Returns a value as an output parameter

I have a question in C where I need to insert coefficients of a quadratic equation into a function and return the number of solutions and result.

Write a program that accepts a series of 3 real numbers, which are the coefficients of a quadratic equation, and the program will print out some solutions to the equation and the solutions themselves. Guidelines:

• Functions must be worked with one of the functions that returns the number of solutions as a returned value, and returns the solutions themselves through output parameters.
• 3 numbers must be received each time. The input will be from a file (will end in EOF)

In the meantime I built the function without reading from a file just to see that it works for me, I built the function that returns the number of solutions but I got entangled in how to return the result as output parameter here is my code for now:

int main ()
{

    double a, b, c, root1,root2,rootnum;

    printf("Enter coefficients a, b and c: ");

    scanf("%lf %lf %lf",&a, &b, &c);

    rootnum=(rootnumber(a,b,c);

    printf("the number of roots for this equation is %d ",rootnum);
}


int rootnumber (double a,double b, double c)
{

    formula=b*b - 4*a*c;

    if (formula<0)

        return 0;

    if (formula==0)

        return 1;
    else
        return 2;
}