Question 309331
{{{a1=-4}}}
{{{d=5}}}
The sum of an arithmetic series is,
{{{Sn=n((a1+an)/2)}}}
Solve for n using the final term. 
{{{an=a1+(n-1)*d=91}}}
{{{-4+(n-1)(5)=91}}}
{{{5(n-1)=95}}}
{{{n-1=19}}}
{{{n=20}}}
Then
{{{S20=20((-4+91)/2)}}}
{{{S20=870}}}