How to write this Lisp / Scheme code?

Question

A lambda expression which takes a function (of one argument) and a number, and applies the function to twice the number.

Answer 1

Applying the function to twice the number:

(lambda (f x) (f (* 2 x)))

Applying the function to the number twice (which is what you may have intended to ask):

(lambda (f x) (f (f x)))

Answer 2

Greg's answer is correct, but you might think about how you might break apart this problem to find the answer yourself. Here is one approach:

; A lambda expression
;(lambda () )

; which takes a function (of one argument) and a number
;(lambda (fun num) )

; and applies the function
;(lambda (fun num) (fun num))

; to twice the number
;(lambda (fun num) (fun (* 2 num)))

((lambda (fun num) (fun (* 2 num))) + 12)