I am having difficulty with converting nested sigma notations into closed forms.

sum [from i=3 to n] (i * sum [from j=i+1 to n] (3) )

I apologize for the notation but I hope I portrayed the question.

The inner one I thought that it would be i(n- (i+1) +1)3 = 3i(n-i) {which I thought is the closed form for a constant} then substitute into the outer one.

Any help or explanation muchly appreciated.

Ray