Answer
$6600$
Work Step by Step
Formula to calculate the sum of an arithmetic sequence is: $S_n=\dfrac{n}{2}(a_1+a_n)$ ...(1)
Here, $a_1: $ First term and $d=$ common difference
Since, we have $d=a_2-a_1=-9-(-15)=6$
and $a_{50}=-15+6(50-1)=279$
Then, our first (1) equation becomes: $S_{50}=\dfrac{50}{2}(-15+279)=\dfrac{50}{2}(264)=6600$
