<s>[INST] Find $2^2 + 5^2 + 8^2 + ... + (3n-1)^2 $ 
First prove that $1^2 + 2^2 + 3^2 +...+ n^2 = \frac{n(n+1)(n+2)}{6}$, then find $$2^2 + 5^2 + 8^2 + ... + (3n-1)^2.$$

I can prove the first part but I have no idea about the second part.
 [/INST] Hint
$$\sum(3k-1)^2=9\sum k^2-6\sum k+\sum 1.$$
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