Inverse functions: f inverse of 11 in one step (IB Maths AA)

The inverse of f undoes f, so f inverse of 11 is simply the input that f turns into 11. With f(x) = 2x + 3 that means solving 2x + 3 = 11: take away 3 and halve, and x = 4. You never need the formula for the inverse function to answer it.
Check it by going forwards: f(4) = 2 times 4 plus 3 = 11. The slips to avoid are working forwards and giving f(11) = 25, stopping at 8 before halving, or reading the minus one as a power and writing 1/f(11).
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