How can I enter the total_area formula in a c++? It shows these error

  area, c++, dev-c++, double

I am a newbie in c++ and I am quite confused in programmer – defined functions.
It shows these errors
40 [Error] cannot convert ‘double’ to ‘double(double, double)’ in assignment
40 [Error] assignment of function ‘double total_area(double, double)’

I cannot enter total_area = cross_area + side_area;
If i try to remove the double, it results to more errors
I cant find any information in youtube taht seems helpful or wvwn google

// P33_2.cpp This program illustrates the local and global variables and call-by-value.
// This program computes the side area and the cross section area of a cylinder
using namespace std;
const double PI = 3.14159; // This variable is defined globally, known to all functions in this program as PI
double cross_area(double r); // Function prototype for function cross_area
double side_area(double r, double h); // Function prototype for function Side_area
double total_area(double cross_area, double side_area);
int main(void)
    double h, r; //variables local to the main function
    cout << "Enter the radius and the height of the cylinder in Cm <Enter> ";
    cin >> r >> h;
    cout << endl;
    cout << "Before I do any computation or call any function, I want to let you know that n";
    cout << "you have entered r = " << r << " and h = " << h << "." << endl;
    cout << "I am planning to use inch, thus in the first function, I will convert r, and " << endl;
    cout << "in the second one I will convert h n";
    cout << "The cross section area of the cylinder is " << cross_area(r) << " inch-sqr endln";
    cout << "The side area of the cylinder is " << side_area(r,h) << " inch-sqr nn";
    cout << "The total surface area is "<< total_area << "inch-sqr n n";
    return 0;
    double cross_area(double r)
    //Cross secion area includes the disks at the bottom and the top
    r = r * 0.3937; // converting r to inch
    return 2*PI*pow(r,2);
    double side_area(double r, double h)
    double area; //variable local to Side_area function
    h = h * 0.3937; // converting h to inch
    area = 2*PI*r*h;
    return area;
    double total_area(double cross_area, double side_area)`enter code here`
    total_area = cross_area + side_area;
    return 0;

Source: Windows Questions C++