How do I find the general term a(n)?

2, 8, 18, 32, 50

I know a(n)=$\displaystyle 2n^2$

But how do I get that?

I know you find the first order difference, b(n)

b(n)= 6, 10, 14, 18 = $\displaystyle 4n-2$

$\displaystyle \sum_{k=1}^{n-1} 4k-2 $

$\displaystyle =2+2n(n-1)-2(n-1)$

Obviously I messed up on that last step. How do I correctly do it? Thanks.