You can use the generic representation of a term of an arithmetic sequence:
[tex]a_n=a_1+d(n-1)[/tex]
Filling in the values a₁=-8, d=5, you have
[tex]a_n=-8+5(n-1)\\a_n=-13+5n[/tex]
Then the value of n for the last term can be found as
67 = -13 + 5n
80 = 5n
16 = n
and the sum can be written as
[tex]S_{16}=\sum\limits_{n=1}^{16}{(-13+5n)}[/tex]
