The formula to find any term of an an arithmetic sequence is given by
\(a _{n}\)=\(a _{1}\)=(n-1)d,
where we have first term \(a_{1}\)=8,
and d which is the common difference , and n is the term we need to find (in our case is 20)
the difference d=3, as you see each number is the previous minus 3
for S20=8+(20-1)(-3)=8+(19)(-3)=8-57=-49
S20=-49, this the 20th term
