It's the function of function: f(g(x))
1st you replace x (of f(x) by the value of g(x)
Example: let's take f(x) = x² ag g(x) = 1/x, you just plug 1/x into x in f(x)
f(g(x)) = f(1/x)² & f(x) =1/x²
If we follow the same methodology we find that f(x) =2/x & g(x) =2/x is the answer to the problem. Proof:
f(g(x)) = f( 2/(2/x)) = f(2(x/2) =2x/2 = x