Answer
$477$
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.
Thus the sum:$\frac{n(a_1+a_n)}{2}=\frac{n(a_1+a_1+(n-1)d)}{2}=\frac{9(12(1)-7+12(1)-7+(9-1)12)}{2}=\frac{9(106)}{2}=477$
