I just need a hint to solve the problem. The method used is illustrated in the example problem as follows: I am supposed to imitate the method used in the example problem to solve the challenge problem in the book as follows Imitate the method used in the last problem to find a formula for the sum 12 + 22 + 32 + ... + k2 when k is a positive integer. I have tried various things like (a+1)4 - a4, (a+1)3 - a3, triangle numbers, odd numbers, but I havent been able to solve it. I just need a hint. This is not a homework problem. This is self study.