In order to create a function where f(3) = 12, you need to establish a relationship between 3 and some number that result in 12.
Some examples are 3*4 = 12, 3 + 9 = 12, 3^2 + 3 = 12
In each of the above cases, simply replace the 3 with 'x', and you'll have a valid answer.
i.e. f(x) = 4*x, f(x) = x + 9, f(x) = x^2 + x
