Given that f(x) = 2x + 11: if f(x) = y = 21, then 21 = 2x + 11, and so 10=2x and x=5.
Thus, f(5) = 2(5) + 11 = 21; (5,21) is a point on the graph of f(x).
If that is so (which it is), then (21,5) is a point on the graph of the inverse function, and 5 is the appropriate output value.
