Try induction!
Good. Be careful with your notation - be as clear as possible, don't put more than one equal sign in your expression!
So you have .
Now, to prove that , consider
and add it to the same sum, written backwards :
.
Add them vertically, term by term. What do you get?
Oh ok. I have one question. Since the problem has 2 in front of the sigma notation in the first term, could i cancel the two out to get sum from {i=1} to {n} n^2 + n - sum from {i=1} to {n} n?
And then I could cancel one n out, which would give me
sum from {i=1} to {n} n^2?
I think that is how you prove it, but I cannot be sure