is there a way to have a (function minus/plus a double variable) as an input?

Question

double a(double x)
{
return area(x,f)+0.5;
}

double b(double x)
{
return area(x,f)-0.5;
}

double rand_normal(void)
{
double r,x,z;
    srand(time(NULL));
    r = rand(