Just call the function by passing the parameters a, b, and c.  Syntax:
retval = function_name(parameter1,parameter2,parameter3); //pass parameters as required
Like this:
int main(void)
{
    float a, b, c;
    double d;
    printf("Enter the values of 'a','b' and 'c': ");
    if (scanf("%f %f %f",&a,&b,&c) == 3)
    {
        d = my_function(a, b, c);
        printf("Result: %f\n", d);
    }
    else
        printf("Oops: I didn't understand what you typed\n");      
}
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solved How to pass variables onto another function? [closed]