Answer
$0\leq x$, $f^{-1}(x)=\sqrt{4-x}$.
Work Step by Step
To make the function one-to-one, according to the graph we can restrict its domain to $0\leq x$. If we want to get a formula for the inverse function, we swap $x$ and $y$ and solve for $y$. Hence, after swapping $x$ and $y$, we get:
$x=4-y^2\\y^2=4-x\\y=f^{-1}(x)=\sqrt{4-x}$
