Challenge 1: A triple Function via Partial Application — Possible Solution
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Main.hs:

multiply :: Int -> Int -> Int
multiply x y = x * y

triple :: Int -> Int
triple = multiply 3

main :: IO ()
main = print (triple 7)

Output:

    21

Explanation:
`triple = multiply 3` partially applies multiply with just its first
argument. Because multiply is really a curried chain
(Int -> (Int -> Int)), supplying only 3 doesn't produce an error or
require a placeholder for the missing argument -- it produces a
genuine, complete, real function of type Int -> Int, which is exactly
triple's own declared type. No lambda (\x -> multiply 3 x) was needed
anywhere.

WHY THIS WORKS AS AN ANSWER
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This uses partial application directly, exactly as the chapter's own
addFive example demonstrates, producing a real named function from a
more general one with no wrapper lambda written at all.