Найти значение функции и её производной при заданном значении x

#include <iostream>
#include <cmath>
 
 
int main()
{
    double x;
    std::cin >> x;
    std::cout << "y = " << 4 * std::pow(x + 1, 3) + 5 * std::pow(x - 1, 5) + 2;
}

#include <iostream>
#include <functional>
 
double find_derivative(
    const std::function<double(double)>& f,
    double x,
    double delta
);
 
int main()
{
    const double eps = 1e-7;
    std::function<double(double)> f = [](double x)
    { return 4. * std::pow(x + 1., 3.) + 5. * std::pow(x - 1., 5.) + 2.; };
 
    double x;
    std::cin >> x;
    std::cout << "y(" << x << ") = " << f(x)
        << "\ny'(" << x << ") = " << find_derivative(f, x, eps) << "\n";
    
    return 0;
}
 
double find_derivative(
    const std::function<double(double)>& f,
    double x,
    double delta
)
{
    double up = f(x + delta);
    double down = f(x - delta);
    double step = 2. * delta;
    double derivative_in_point_x = (up - down) / step;
    return derivative_in_point_x;
}