Let a(n) denote the nth term in the sequence. Then
a(5) = a(4) + d
a(6) = a(5) + d = a(4) + 2d
a(7) = a(6) + d = a(4) + 3d
a(8) = a(7) + d = a(4) + 4d
where d is the fixed difference between consecutive terms in the sequence.
We have a(4) = 65 and a(8) = 45, so we solve for d:
45 = 65 + 4d ==> d = -5
so, given some term in the sequence a(n), the next term is a(n + 1) = a(n) - 5.
Continuing the pattern above, we would wind up getting
a(31) = a(4) + 27d
(In case the pattern isn't clear, watch the coefficient of d; then 1 + 4 = 5, 2 + 4 = 6, 3 + 4 = 7, and so on.)
Then the 31st term in the sequence must be
a(31) = 65 + 31(-5) = -90