To get f-1 rewrite f as:
\(f=\frac{a}{x+2}\\x+2=\frac{a}{f}\\x=\frac{a}{f}-2\)
Now change f into x and x into f-1:
\(f^{-1}=\frac{a}{x}-2\)
We have: \(f(0)=\frac{a}{2}\text{ and }f^{-1}(3a)=\frac{a}{3a}-2\rightarrow-\frac{5}{3}\)
I'm sure you can take it from here!
