Let a and B be set n of natural numbers, Mapping f: A to B maps element n in set a to element 2 ^ n + N in set B, then under mapping f, the original image of image 11 is

Let a and B be set n of natural numbers, Mapping f: A to B maps element n in set a to element 2 ^ n + N in set B, then under mapping f, the original image of image 11 is

11=2^n+n,n=3
This can be substituted one by one. It's not easy to solve it