factor completely
Z^2 - 18Z + 81 - 25x^2
The first three terms are a perfect square, factor that.
Now, you have a new first term squared minus 25x^2, which is the difference of two squares.
Z^2 - 18Z + 81 - 25x^2
Now, you have a new first term squared minus 25x^2, which is the difference of two squares.