Problem:

a^2-9 a^2-3a
----- * --------
a^2 a^2+a-12
Here Is What I Got:
(a+3)3a
-------
(a+4)
*Please Check It.I think I did it wrong though.. =[

I'm going to assume your problem is this:

(a^2-9)/a^2 * (a^2-3a)/(a^2+a-12)

Note: I'm using * to mean multiply.

If that is the problem, then you want to try to factor first, so you can reduce wherever you can.

Factoring:
(a+3)(a-3)/a^2 * a(a-3)/(a+4)(a-3)

Cancel out (a-3) in both the numerator and denominator of the second fraction to end up with this:

(a+3)(a-3)/a^2 * a/(a+4)

Multiplying:
a(a+3)(a-3)/a^2(a+4)

We can still reduce a/a^2 to end up with this:
(a+3)(a-3)/a(a+4)

And that's about as far as we can go on this one!

I hope this helps and is what you were asking for this problem.