Chapter 5.2B, Problem 24A

In each of the following, find the next two terms. Assume the sequence is arithmetic or geometric, and find its common difference or ratio and the $n\text{th}$ term.

a. $10,7,4,1{,}^{-}2{,}^{-}5,\dots$

b. ${}^{-}2,4{,}^{-}8,16{,}^{-}32,64,\dots$