Je n'arrive pas a separer mon code

(fichier .cpp)
#include "math.h"

int carer(int a)
{
     int b;
     b = a*a;
     return b;
}


(fichier .h)
#ifndef MATH_H_INCLUDED
#define MATH_H_INCLUDED

int carer(int a);

#endif


(le programme)
#include <iostream>

using namespace std;

int main()
{
    cout << "Hello world!" << endl;
    int d;
    d = 5;
    int m = carer(d);
    cout <<"le carer de 5 est :"<<m<<endl;
    return 0;
}