\[ax = - 8;\ \ \ x - натуральное\ \]
\[число;\]
\[x = - \frac{8}{a}\]
\[a = \{ - 1;\ - 2;\ - 4;\ - 8\}.\]
\[Ответ:\ a = \{ - 1;\ - 2;\ - 4;\ - 8\}.\]