just expand it out to start with
$4A -Ax + 3x = 9B + 2Bx$
for an identity this must hold true for all x. So equate the constant bits and equate the bits multiplying $x$.
Do you understand this? If not stop and look at it until you do.
Now we have two equations and 2 unknowns and use your favorite method to solve this system of equations.
You should know how to do this so I'm going to leave it to you to finish.