I have a function

If the domain of this function is restricted to 0 < x < 8/3 then f-1(x) can be defined - ie inverse function.

I am told to find the gradient of at the point ( )

My first thought was to work out the inverse function. But this looks hard? Is this what I should do? Or is there another way? I know that an inverse function is a reflection of function on y=x - can I somehow use this to calculate? If so how?

Angus