Shouldn't it be ?
it may be easier to write
Which is a quadratic, so
and hence the inverse is
(not sure if you saw my previous post before i edited it to include the answer)
using your formula you should get the same answer as i posted:
multiple everything by f^-1(x):
this is a quadratic in . the coefficients (a,b,c) are (-1,-x,2)
Solve with quadratic formula