Question 600203
It looks like you want to factor here. 


x^2-18x+81-y^2


(x^2-18x+81)-y^2


(x^2-9x-9x+81)-y^2


((x^2-9x)+(-9x+81))-y^2


(x(x-9)-(x-9))-y^2


(x-9)(x-9)-y^2


(x-9)^2-y^2


[(x-9)-y][(x-9)+y]


(x-9-y)(x-9+y)



So x^2-18x+81-y^2 completely factors to (x-9-y)(x-9+y)