if it wasn't for the word INDUCTION
n^5 + n^3 +2n
=n^5 + n^3 -2n +4n
= n(n^4 +n^2 -2) + 4n
=n(n+1)(n-1)(n^2 + 2) +4n
now if n is even, then n^2, n^2 +2 will be even
the product n(n^2 + 2) will be the product of 2 even numbers, hence a multiple of four
if n is odd, then (n-1)...