My 2 cents on this question:

We can write 3f(x) = 4f(x) - f(x) and move the -f(x) to the right hand side.

Let's simplify 4f(x) first, and ultimately the entire left hand side.

so

Now we need to combine this with f(4x^3 -3x):

Let

We need:

Remember that

So we have:

The left hand side argument, assuming 1-1, should equal the argument of the right hand side, x.