by the addition-subtraction method.
Moderator edit: Approved Challenge question.
A simpler method would help avoid algebraic mistakes (btw please recheck your algebra as it needs fixing).
Since the A-S method hasn't been applied, my challenge still stands. You can try again TheCoffeeMachine or maybe someone else out there wants to take a whack at this problem (refer to the third post on this thread).
Since the A-S method hasn't been used to solve this problem, my challenge still stands and if you want to see details about this method, I'll just refer you again to the third post on this thread.