Extreme beginner quadratic roots solver

Right, so I am brand new to programming (doing it for a module at uni). We have to write a programme to find real solutions to a quadratic to 3 decimal places. We need to use command line arguments and the if-else construct.

Looking over various code I have found online and the very little that I actually do understand, I put this together and would like to understand what it all means I have made bold the bits I am unsure about.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(int argc, char* argv[])
{
     double  a, b, c, valida, validb, validc, x1, x2;
     int     valid_input;
	 
     if(argc != 4)								
     {
        printf("Enter three arguments\n");
        exit(EXIT_FAILURE);
     }
	 
     else
     {
        valida=sscanf(argv[1], "%lg", &a);
        validb=sscanf(argv[2], "%lg", &b);
	validc=sscanf(argv[3], "%lg", &c);

	valid_input = (valida != 0) && (valida != EOF);
        valid_input = valid_input && (validb !=0) && (validb != EOF);
	valid_input = valid_input && (validc !=0) && (validc != EOF);
	
        if(!valid_input)
        {
			printf("Please enter 3 valid digits...\n");
			exit(EXIT_FAILURE);
        }
		
		else
		{
					
			if((a==0)&&(b==0))
			{
				printf("It is not possible to find the roots of these numbers \n");
				printf("as both 'a' and 'b' coefficients are zero\n");
				exit(EXIT_FAILURE);
			}
			
			if(a==0)
			{
				x1 = (-(c/b));
				printf("There is 1 root");
				printf("x1 = %.3lf\n",x1);
			}
			
			else
			{
					if(((b*b)-(4*a*c)) > 0)
					{  
						x1 = ((-b)+(sqrt((b*b)-(4*a*c))))/(2*a);
						x2 = ((-b)-(sqrt((b*b)-(4*a*c))))/(2*a);
					
						printf("There are 2 roots\n");
						printf("x1 = %.3lf\n",x1);
						printf("x2 = %.3lf\n",x2);
						exit(EXIT_SUCCESS);
					}
		
					if(((b*b)-(4*a*c)) == 0)
					{
						x1 = ((-b)/(2*a));
					
						printf("There is 1 root\n");
						printf("x = %.3lf\n",x1);
						exit(EXIT_SUCCESS);
					}
		
					else
					{
						printf("Cannot calculate imaginary roots\n");
						exit(EXIT_FAILURE);
					}	
					
					
			}
		}
	 }
	 
	 
	 return(0);
}

For the first bold bit;
-Should I be using float or double?
-Is it right to set valid_input as an integer?
-Should I define a,b,c,x1,x2 before main()?

For the second (validation);
-Is %lg correct? and if using float, it should be %f, right?
-In the valid_input bit, have I said that it is only valid if ALL 3 lines are met or is each one separate (as in is it only invalid if a,b and c are zero, or if any of them are)?

For the third bit;
-Is %.3lf correct for 3 decimal places?
-Would it be different if it was a float? %.3f?
-Does it need an exit(EXIT_SUCCESS) after it?

In general;
-Should I define the discriminant before main() and then use it in the if-else instead?

---------------------------

I apologise if I have gone against any board rules, I have read the FAQ and searched the forum before posting and tried to follow the guidelines....

Thanks

Float should be fine. Int is correct, logical statements return 1 or 0 as signed integers. Define your variables inside of main() like you already have it.

Should be %.3f for a float, I believe.

The exit() function is more for when errors are encountered, as I understand it. Should use return instead.

The sscanf() parts you highlighted are used to just locate and put the right values from the *argv[] (a 2 D array), into the correct variables: a, b, or c

sscanf() is scanf() used on a designated string.

.3lf is the printf() format for a double, printed out to 3 decimal places.

Thanks, so

valida=sscanf(argv[1], "%f", &a)

would assign the first command line argument to the variable a in the programme....

but what about thei valida? is that another variable I have created? or does that mean something like, "to validate a the assignment of it must be a float?