double sum;
for(int i = 0; i < 13; i++)
{
  for(int j = 0; j < 13; j++)
  {
    sum = 0;
    for(int k = 0; k < 7; k++)
    {
      sum + = x[i][k] * y[k][j];
      res[i][j] = sum;
    }
  }
}