Question 98008
Divide and conquer! Split this up and you will get it.

{{{x^2-2*x*y+y^2}}} is the first part. This is a simple expansion of
{{{(x-y)^2}}}

The second part: {{{a^2-2*a*b-b^2}}} is a simple expansion of
{{{(a+b)^2}}} if you first factor out a -1, which is already in the problem.
{{{-1*(a^2+2*a*b+b^2)}}} This is done when you put your parentheses around the second polynomial {{{a^2-2*a*b-b^2}}}
So, the factored version is 
{{{(x-y)^2-(a+b)^2}}}