Answer
$S_{14}=-210$
Work Step by Step
The first n term of an arithmetic sequence can be calculated as:
$S_n=\frac{n}{2}(a_1+a_n)$
We have to find $a_{14}$ in order to find the sum.
First, $d$ can be calculated as the difference of subsequent terms:
$d=a_2-a_1=7-11=-4$
$a_{14}=a_1+ d(14-1)=11+(-4)\times13=11-52=-41$
By substituting in the formula of the sum, we get:
$S_{14}=\frac{14}{2}(11+(-41))=7\times(-30)=-210$
