Question 21994
First step is to use FOIL
First
Outer
Inner
Last

You should have this:
{{{x^2-xy+xy-y^2}}}
Answer:
{{{highlight(x^2-y^2)}}}
HINT:
If you know the difference of squares:
{{{x^2-y^2}}}={{{(x-y)(x+y)}}}
It'll make math easier and quicker, so try to remember that. 
Hope this helps!