Answer
$${a_8} = - x + 61$$
Work Step by Step
$$\eqalign{
& {a_1} = 6x - 9,\,\,{\text{ }}\,{a_2} = 5x + 1 \cr
& {\text{Find }}d \cr
& d = {a_2} - {a_1} \cr
& d = \left( {5x + 1} \right) - \left( {6x - 9} \right) \cr
& d = 5x + 1 - 6x + 9 \cr
& d = - x + 10 \cr
& {\text{The general term is}} \cr
& {a_n} = {a_1} + \left( {n - 1} \right)d \cr
& {a_n} = 6x - 9 + \left( {n - 1} \right)\left( { - x + 10} \right) \cr
& \cr
& {\text{Finding }}{a_8} \cr
& {a_8} = 6x - 9 + \left( {8 - 1} \right)\left( { - x + 10} \right) \cr
& {a_8} = 6x - 9 + 7\left( { - x + 10} \right) \cr
& {a_8} = 6x - 9 - 7x + 70 \cr
& {a_8} = - x + 61 \cr} $$
