How can i factor a polynomial with multiple variables

I have to factor:
x^2 - 6cx + 9c^2 - y^2. I learned it in class but don't remember how to do it and my notes don't help. Please help?

x^2 - 6cx + 9c^2 - y^2. = (x-3c)^2 -y^2 = (x-3c-y)(x-3c+y)

x^2 - 6cx + 9c^2 - y^2

{ x^2 - 6cx + 9c^2 } - y^2

( x - 3c )^2 - y^2.....................A^2 - B^2 = (A + B)(A - B)

{ (x - 3c) + y }*{ (x - 3c) - y }

(x - 3c + y)(x - 3c - y).................ANSWER

The left three terms become: (x - 3c)^2

So we now have:

(x - 3c)^2 - y^2 =

(x - 3c - y)*(x - 3c + y)