So i have done and got I have then got simplified and am now stuck, any help please? Thanks.
If you are trying to find an a such that f_a is its own inverse:
Clear the complex fractions:
Now put the whole equation under a common denominator, cancel the denominator, and this leaves you a polynomial in powers of x. You will see that the x^4 coefficient cannot be set to 0, so this problem cannot be solved for any value of a.