Algebra 2 help?

=(x^2-2xy+y^2)-9

[ (x - y) - 3 ] [ (x - y) + 3 ]

Note that    (a - b)(a + b)  =  a² - b²

And for this problem we can say that  a = (x - y)  and  b = 3 .

So.....

[ (x - y) - 3 ] [ (x - y) + 3 ]  =  (x - y)²  -  3²

[ (x - y) - 3 ] [ (x - y) + 3 ]  =  (x - y)²  -  9