Find the sum 3+6+9+⋯+3n3+6+9+\dots+3n3+6+9+⋯+3n.
3n(n+1)/23n(n+1)/23n(n+1)/2
3n2/23n^2/23n2/2
3n(n−1)/23n(n-1)/23n(n−1)/2
3(n+1)3(n+1)3(n+1)