Ijust need someone to check my

what is the sum of the arithmetic sequence 3, 9, 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