Two real numbers $a , b$ greater than 1 satisfy
$$\log _ { a } b = 3 , \quad \log _ { 3 } \frac { b } { a } = \frac { 1 } { 2 }$$
What is the value of $\log _ { 9 } a b$? [3 points]\\
(1) $\frac { 3 } { 8 }$\\
(2) $\frac { 1 } { 2 }$\\
(3) $\frac { 5 } { 8 }$\\
(4) $\frac { 3 } { 4 }$\\
(5) $\frac { 7 } { 8 }$