subtracting fractions in classes

Hi

I am having difficulty in creating a function for subtracting fractions in a class. I was given this advice to do it

Note:subtraction can be achieved by adding (by changing the sign of the numerator - this can be achieved by multiplying it by -1).

and my attempt is shown below:

Code:

void Fraction::subtract(Fraction f2, Fraction& f3)
{
  int s_denom = denom * f2.denom ;
  int s_num = ((num * -1) * f2.denom) - ((f2.num * -1) * denom);
  f3.create(s_num,s_denom);
  f3.simplify();
}

but a subtraction seems to come out a little wrong for example the output is:

Code:

Enter first numerator: 3

Enter first denominator: 4

Enter second numerator: 2

Enter second denominator: 4

3/4 - 2/4 = -1/4

Can anyone help me to correct this little error.
Thanks if you can.

Code:

void Fraction::subtract(Fraction f2, Fraction& f3)
{
  int s_denom = denom * f2.denom ;
  int s_num = num * f2.denom - f2.num  * denom;
  f3.create(s_num,s_denom);
  f3.simplify();
}

Multiply f2.num by -1 and add the products, or don't bother multiplying by -1 and subtract the second product from the first. Either way, you shouldn't multiply num by -1.