Hint: Here are the formulas you need to know:

(1) The change of base formula:

(2) Cor. of (1): ....(by taking in part (1))

and are general terms here, not the and in your problem.

Apply the above formulas in the order they are given and you will have your proof. just to give you a direction (the manipulation can work in either) start with the left hand side of your equation