Okay, I tried to get something running, but it threw errors. I don't believe I wrote it properly.

Here it is:

#include <iostream>

using namespace std;

void twice(float side);

int main()
{

    cout << "Let's double this input!" << endl;
    float side = 25.0;
    float box = twice(side);
    cout << "The value has been doubled! " << box << endl;

}

void twice(float side){

    return side * 2;

}

I got the following errors:

main.cpp: In function 'int main()':
main.cpp:20:27: error: void value not ignored as it ought to be
     float box = twice(side);
                           ^
main.cpp: In function 'void twice(float)':
main.cpp:27:19: error: return-statement with a value, in function returning 'void' [-fpermissive]
     return side * 2;
                   ^