Answer
$a_{90}=-248$
Work Step by Step
The $n^{th}$ term of an arithmetic sequence can be found using the formula:
$a_n = a_1 + (n-1)d$
where
d = common difference
$a_1$ = first term
$\\a_n= n^{th} \text{ term}$
The given arithmetic sequence has:
$a_1 = -70
\\d=-2$
Use the formula above to find the 90th term:
$a_n=a_1+(n-1)(d)
\\a_{90}=-70 + (90-1)(-2)
\\a_{90}=-70+89(-2)
\\a_{90}=-70+(-178)
\\a_{90}=-248$