I just need someone to check my answer:

What is the sum of the arithmetic sequence 3, 9, 15 ..., if there are 24 terms?

a) 1452

b) 1728 (my answer)

c) 2028

d) 2268

Sum is n (an+a1) / 2

an=a1+d (n-1)

common differnce is 6

first term si 3

an=3+6 (n-1)

24th term

a24=3+6 (24-1)

a24=3+6 (23)

a24=3+138

a24=141

so nth term is 141

n=24

24 (3+141) / 2=12 (144) = 1728

answer is B