The long way:
(x^2 + y^5)(x^2 - y^5)
FOIL
x^4 - x^2y^5 + x^2y^5 - y^10
x^4 - y^10
The short way (once you're used to it):
Whenever you have something like (a+b)(a-b), you end up with a^2 - b^2.
In (x^2+y^5)(x^2-y^5) the x^2 is like the a and y^5 is like the b.
You end up with (x^2)^2 - (y^5)^2 or just x^4 - y^10