Answer
$a_{80} = 157$
Work Step by Step
The $n^{th}$ term of an arithmetic sequence is given by the formula:
$a_n = a_1 + (n-1)d (1)$
where
$a_1 = \ First \ Term; \\ d = \ Common \ Difference$
We have: $a_1=-1 \\ d=a_2-a_1=3-1=2$
We will substitute the above data into formula (1) to obtain:
$a_n=-1+(2) (n-1)$
In order to compute the $80th$ term, we need to plug in $80$ for $n$ in the above form to obtain:
$a_{80} = -1+(2) (80-1) =-1+(79)(2)$
So, $a_{80} = 157$