This is an arithmetic sequence because there is a common difference of 3, each term is 3 greater than the previous term.
The sum of any arithmetic sequence is the average of the first and last terms times the number of terms...mathematically this winds up as:
s(n)=(2an+dn^2-dn)/2, a=initial term, n=number of terms, d=common difference
In this case:
s(n)=(-8n+3n^2-3n)/2
s(n)=(3n^2-11n)/2 so
s(90)=(3*8100-11*90)/2
s(90)=11655