Answer 1

for (int i = 0; i < N; i++)

    {

        for (int j = 0; j < N; j++)

        {

            res[i][j] = 0;

            for (int k = 0; k < N; k++)
res[i][j] +=
mat1[i][k]* mat2[k][j];

        }

    }