Chapter 1.CR, Problem 5CR

Find a possible $n\text{th}$ term in each of the following:

a. $5,8,11,14,\mathrm{...}$

b. $3,9,27,81,243,\mathrm{....}$

c. $2^2-1,2^3-1,2^4-1,2^5-1,\mathrm{...}$