Simplify by removing factors of 1.
z^2-6z+9/z^2+2z-15
[z^2-6z+9]/[z^2+2z-15] factor top and bottom
[ ( z -3) (z - 3) ] / [ ( z + 5) ( z - 3) ] cancel (z - 3) on top/ bottom
( z - 3) / ( z + 5)
