The most important clue about proving this problem is the following:
$ Py(Y) = Py(Y = k) = Py(X = N - k) = Px(N - k) $
We know the binomial expression for X because it's stated that it has params (N, p), so we can just substitute what we found in the previous step for N and p and try to get the resulting expression in the form of Y as a binomial variable with params (N, 1-p).
