Good afternoon everyone !

I'm supposed to calculate this sum !

I know that in the end, i'm supposed to get something like : (n(n+1)(4n-1))/6 or if you dev. this expression : 5n/6...

Could anybody give me the step-by-step resolution of this exercice ? I have really no idea about what i'm supposed to do with it. I tried to transform it into a double sum, but that's all...