Can someone tell me if this answer is correct or not?

Question

anonymous · Answer

What are the factors of x2 - 81?
I put prime

nikato · Answer

IM BACK! haha

and no, its not prime.

nikato · Answer

its a special kind called difference of square.

see how 81 is a perfect square because 9^2=81

anonymous · Answer

Ah, okay, so would the answer be (x-9) (x-9) then?

nikato · Answer

(x-9)(x+9)

remember with difference of squares theres is one - and the other is +

nikato · Answer

and you can check.
(x-9)(x-9)= x^2 -18x+81

anonymous · Answer

I did (x-9)(x+9) and that didn't work for me. Are you sure? I got 

x^2-9x+9x-81 

After this point I had to either add the -9 to itself and positive 9 or the other way around which wouldn't equal x^2=-81

pawanyadav · Answer

x^2-y^2= (x+y)(x-y) always.
In your question y=9.

anonymous · Answer

Wait so do you always have to cross out the two x that isn't squared or cubed?

nikato · Answer

ys. because they cancel each other. they add up to 0

nikato · Answer

well, only in this problem

anonymous · Answer

Ooooh, okay. That makes allot more sense now. Haha, thanks for telling me.