you have 1*1, 2*2, ... n*n, (n+1)*(n-1), (n+2)*(n-2), ..., 1*(2n-1)
now if you take the first and the last and continue from there you have [1+(2n-1)]*1, [2+(2n-2)]*2, ... [(n-1)+(2n-{n-1})]*n-1, n*n
again you can simplify that to 2n*sum(1,...,n-1) + n*n = n * ( [2 * {(n-1) * n / 2}] + n ) = n^3
so should enable you to solve your problem