posted by Adalberto .
Sharon Spent $2 on a magazine; then she spent half of the rest of her money on shoes. She spent half of her remaining money on lunch. She has $12 left. How much did Sharon start with?
since you don't define a,b it's hard to say.
You can check by plugging in the results you got to see whether they satisfy the equation you derived.
One thing that troubles me is that X vanishes from the equation if you subtract X from both sides.