Question 688914
n^2+3n = n(n+3)


Case 1: if n is even, then 2 is a factor of n(n+3) or n^2+3n (since n is a factor of n^2+3n)


Case 2: If n is odd, then n+3 is even (odd+odd = even), so 2 is factor of n+3 which means 2 is a factor of n^2+3n