You first switch f(x) with x in this equation
<span>x=<span>1<span>9f(x)−2</span></span></span>
Then you solve for f(x).
<span>9f(x)−2=<span>1x</span></span>
<span>9f(x)=<span>1x</span>+2</span>
<span>f(x)=<span>1<span>/9x</span></span>+<span>2/9</span></span>
Then you simply repalace f(x) with f^-1(x)
It will look like this
<span><span>f−</span>1(x)=<span>1<span>/9x</span></span>+<span>2/<span>9 Hope i could help </span></span></span>