Answer
$225$
Work Step by Step
The sum of the first $n$ terms of an arithmetic sequence can be obtained by the following formula: $\frac{n(a_1+a_n)}{2},$ where $a_1$ is the first term, $a_n$ is the nth term and $n$ is the number of terms.
The nth term of an arithmetic sequence can be obtained by the following formula: $a_n=a_1+(n-1)d$, where $a_1$ is the first term and $d$ is the common difference.
Hence here: $d=3,n=15,a_1=-6,a_{15}=-6+(15-1)3=-6+42=36$
Thus the sum:$\frac{15(-6+36)}{2}=450/2=225$
