Local variables aren't initialized to anything in particular, so there is no way to predict what the value of c will be after this statement. Move it down in your code to a place after you give values to a and b but before you print.
These don't do anything of use to you. All each statement does is evaluate the contents of a or b and do nothing with it. You can safely remove these two statements.
You must include stdlib.h to make the system function available.
printf("Enter the value of the first number: ");
printf("Enter the value of the second number: ");
c = a + b;
printf("%d + %d = %d\n", a, b, c);