Question 171173
 Formula for an arithmetic series is:


{{{Sn=(A1 + An)*n/2}}}

You have given A1 = -11
               An = 13
               n = 7
substitute and simplify :

{{{Sn = (-11 + 13)* 7/2}}}
{{{Sn = 2*7/2}}}
The answer is:
Sn = 7