\[ax = - 6;\ \ \ x - натуральное\ \]
\[число;\]
\[x = - \frac{6}{a}\]
\[a = \{ - 1,\ - 2,\ - 3,\ - 6\}.\]
\[Ответ:\ a = \{ - 1;\ - 2;\ - 3;\ - 6\}.\ \]