Question 1092090
Start with the starting value, {{{-9}}}.
Then to get each successive term find the amount added, 
{{{-3-(-9)=-3+9=6}}}
So add 6 to each previous value to get the next.
Let's choose k as the index.
So,
{{{-9+6*(k-1)}}} is the formula to get any value.
So for the first value, {{{k=1}}},
{{{-9+6*(1-1)=-9}}}
Second, {{{k=2}}}
{{{-9+6*(2-1)=-9+6=-3}}}
And so on.
So then stick this into the summation notation.
You can simplify too,
{{{-9+6(k-1)=-9+6k-6=6k-15}}}
So,
{{{S[n]=sum((6k-15),k=1,n)}}}