28. There are a few integer values of $a$ such that $\frac{a^{2}-3 a-3}{a-2}$ is an integer. Find the sum of all these integer values of $a$.
28. Ans: 8 . Since $\frac{a^{2}-3 a-3}{a-2}=a-1-\frac{5}{a-2}$, thus $a=-3,1,3,7$. Their sum is 8 .