Find a Formula for the Sum of the First N $9,12,15,18,21 , \ldots$

Multiple Choice

Find a formula for the sum of the first n terms of the sequence. $9,12,15,18,21 , \ldots$

A) $S _ { n } = \frac { n } { 2 } ( 3 n + 15 )$
B) $S _ { n } = \frac { n } { 2 } ( 3 n + 3 )$
C) $S _ { n } = \frac { n } { 2 } ( - 3 n - 15 )$
D) $S _ { n } = \frac { n } { 2 } ( 3 n - 15 )$
E) $S _ { n } = \frac { n } { 2 } ( - 3 n + 15 )$