algebra precalculus - How to reach $dfrac{(n-1)n(2n-1)}{6n^3}$

I am trying to refresh my Maths after a lot of years without studying them, and I am finding a lot of difficulties (which is actually nice). So, my question:

I don't understand the next equality. How to reach $\frac{(n-1)n(2n-1)}{6n^3}$ from $$\frac{1^2+ 2^2+...+ (n-1)^2}{n^3}$$?

Thank you very much (and sorry if I make mistakes; my English is also rusty :))