The series is an arithmetic series with first term -10 and common difference (-2-(-10))=8. The general term a_n can be written as
a_n = -10 + 8(n-1)
or
a_n = -18+8n
The last term corresponds to
110 = -18+8n
128 = 8n
16 = n
So, the summation can be written as
a_n = -10 + 8(n-1)
or
a_n = -18+8n
The last term corresponds to
110 = -18+8n
128 = 8n
16 = n
So, the summation can be written as