One approach is to find g(x), the inverse of f(x):
f(x) =
,
To avoid confusion, I'll use y=f(x), solve for y, and after that, we can write g(x):
f(x) = 
Square both sides

Add 16 to both sides, this completes the square on RHS

Factor RHS
Take square root of both sides

Subtract 4 from both sides, and re-write with x on LHS:
Now we can write
, and g(x) also has domain
g'(x) =
Just plug in x=3 to g(x) and g'(x) to get the desired values ( I got g(3)=1, g'(3)=1/10)