What is the answer to my question 2y^5-4y^5

I'm not actually sure what your question really is, but I'll take a guess. 
Before I go to work on that problem, let me ask you a few other questions:

-- If you have 2 cows and you subtract 4 cows, what do you end up with ?
-- If you have 2 rocks and you subtract 4 rocks, what do you end up with ?
-- If you have $2 in the bank and you subtract $4, what do you end up with ?
-- If you have 2 puppies and you subtract 4 puppies, what do you end up with ?
-- If you have 2x and you subtract 4x, what do you end up with ?

Did you say "negative 2 of them" for each question ?  Good, good.

So now, if you have 2 ' y⁵ 's and you subtract 4 ' y⁵ 's, what do you suppose
you would end up with ?

I'm going to take your points and just let you think about that, because I think
I've given you all the help you need to answer the question on your own.

(That's if I actually understand what your question is.)