fof(x) = x

or, f [ f (x) ] = x

or, f ( a x / x + 1 ) = x

or, a²x / ax + x + 1 = x

or, a²x  =  ax² + x² + x

or, a² - ax - ( x + 1 ) = 0

So, a = x + ( x² + 4x + 4 )^1/2 / 2 = x + (x + 2) / 2

So, a = x + 1, or a = -1

we know, ,        .

apply this rule and u will get thr result. 

hope this helps.