Please help!
You want to make sure that you satisfy these conditions:
$\displaystyle \lim_{x\to-9^-}f\left(x\right)=\lim_{x\to-9^+}f\left(x\right)$
and
$\displaystyle \lim_{x\to7^-}f\left(x\right)=\lim_{x\to7^+}f\left(x\right)$
What will then happen is this: You will have generated a system of equations, and now you can easily solve for a and b.
Can you try to take it from here?
i generated the equations : 8=-9x + b and -8=7x + b and found b to be -1.
However, I do not know how to find a.. =[
EDIT: I think I got it.. I got a and b to both equal -1.
Is that correct?