How do you find the nth term in a sequence of numbers if the values are increasing then decreasing? Ex. 1: -2, 3: 10, 5: -18

Answer 1

There are probably more than one "correct" answer.

1: -2, 3: 10, 5: -18
I think of it as two separate series interleaved together, namely:
1, 3, 5, 7, 9....
and
-2, 10, -18, 26, -34, ....
Your teacher has the final word.