You haven't yet gotten the concept of what f(x) and f( )
mean.

f(x) is the same thing as y, and

f( ) means the value of y when whatever is inside the
parentheses is substituted for x.

 and 



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In other words, yours is the same problem as this:

Given



Find a value of k such that when you substitute 2k
for x, you will get y = 2k.

So you substitute x = 2k and y = 2k



Solve that for k.  You will get two solutions, and one of
them is in the list.

Try to learn to interpret functional notation.
Study that real hard until you get what it's all about:

f(x) is the same thing as y, and

f( ) means the value of y when whatever is inside the
parentheses is substituted for x.
Edwin