$\displaystyle f_a (x) = \frac{2}{x } + \frac{x}{ 2} - a$ So i have done $\displaystyle f(f_a(x)) = x$ and got $\displaystyle \frac{2}{ \frac{2}{x } + \frac{x}{2 } -a } + \frac{\frac{2}{x }+\frac{x}{ 2} -a }{2 } -a = x$ I have then got simplified $\displaystyle \frac{5}{x } - \frac{2}{ a} + \frac{x}{4 } - \frac{3a}{2 } = 0 $ and am now stuck, any help please? Thanks.