Answer
$$\eqalign{
& {a_n} = - \frac{1}{2}n + 3 \cr
& {\text{The domain is }}D:\left\{ {1,2,3,4,5,6} \right\} \cr
& {\text{The range is }}R:\left\{ {0,0.5,1,1.5,2,2.5} \right\} \cr} $$
Work Step by Step
$$\eqalign{
& {\text{From the graph we can see that}} \cr
& {a_1} = \frac{5}{2}{\text{ and }}{a_2} = 2 \cr
& {\text{Find }}d \cr
& d = {a_{n + 1}} - {a_n} \cr
& d = 2 - \frac{5}{2} \cr
& d = - \frac{1}{2} \cr
& \cr
& {\text{The }}n{\text{th Term of an Arithmetic Sequence is given by}} \cr
& {a_n} = {a_1} + \left( {n - 1} \right)d \cr
& {\text{Then}} \cr
& {a_n} = \frac{5}{2} + \left( {n - 1} \right)\left( { - \frac{1}{2}} \right) \cr
& {a_n} = \frac{5}{2} - \frac{1}{2}n + \frac{1}{2} \cr
& {a_n} = - \frac{1}{2}n + 3 \cr
& {\text{The domain is }}D:\left\{ {1,2,3,4,5,6} \right\} \cr
& {\text{The range is }}R:\left\{ {0,0.5,1,1.5,2,2.5} \right\} \cr} $$
