Hi Renee,
Just as with the addition of any number and a fraction you must get a common denominator and then add
so if we had the following
4 + 1/3 We would get a common denominator of 3
and we would have
12/3 + 1/3 = 13/3
In this example the common denominator is (x-y)
so the expression becomes
(x+y)(x-y)/(x-y) - (x2 + y2)/(x-y)
this becomes
(x2 - y2)/(x-y) - (x2 + y2)/(x-y)
combine the numerators
-2y2/(x-y)
Hope this helps