expand completely (X^4-Y)^5
do you just multiply by the exponents?
X^20-Y^5
no
imagine (x-y)^2
it is not
x^2 - y^2
but
x^2 - 2 x y + y^2
You need the "binomial theorem"
If you have learned it your problem is straightforward, follow the recipe.
If you have not had it in your course, I feel for you because you will have to do it step by painful step using the distributive property of multipication.