Find the sum of n terms of the series 2.4+5.7+8.10+------------

Question

$(a)\;\frac{n(n+1)(2n+1)}{6}\qquad(b)\;\frac{(3n+1)(n+1)n}{9}\qquad(c)\;n\qquad(d)\;\frac{n}{2} (6n^2+9n+1)$