If f(a)=K, then which of the following must be true?

f(K)=a
f inverse(k)=a
f inverse(a)=K

2 answers

Properties of inverses:
If f(a)=K
then
1. f-1(K)=a
2. f-1(f(a))=a
... etc

None of the above answers is correct unless the second one says
f inverse(K)=a (i.e. uppercase K)
If ƒ(x ) = 2x, then ƒ -1(x ) =