13. Find all integers $a$ such that the quadratic equation $a x^{2}+2 a x+a-9=0$ has at least one integer root.
(Hint: $a=\frac{9}{(x+1)^{2}} \geqslant 1, -4 \leqslant x \leqslant 2$. Discuss the values of $x$ respectively, we can get $a=1,9$)