The idea is good ! (I wonder how you got it)
however, I can point out two problems :
In your second line, you forgot the part, but I guess it is just a typo.
When substituting different values of , you wrote 6(1), 6(2)... which is false since it is 4i in the formula. Moreover, you forgot each time to add 1.
So adding them together shall give something like :
Edit : and yes, your expression for the sum of the squares is correct =)