return the answer from "void" to another function

I got question how can i return the answer to display because i kept getting an error that void can't return a variable. How can i send it to my display function.

 void Rational::add(const Rational&h2)
    {

        int num = 0;
        int dem = 0;
        add(h2);
        int P = num * h2.dem + h2.num*dem;
        int Q = dem*h2.dem;


    }

    void display() const; // _p:_q
    {
         if (Q == 1) // e.g. fraction 2/1 will display simply as 2
                cout << P << endl;
            else
                cout <<P << "/" << Q << endl;
    }