Question from "How To Pass The GMAT"

Hi, there. I'm happy to help with this.

If x + 3y = 2x - y, then x - 2y is:

Start with the original equation ...
(1) x + 3y = 2x - y
Subtract x from both sides ...
(2) 3y = x - y
Subtract 3y from both sides ...
(3) 0 = x - 4y

This expression does not allow us to determine the value of x - 2y, so the correct answer would be E.

I point out --- if the prompt equation had been...
x + 3y = 2x + y
...then the correct answer would be A.

What is the source? Perhaps they made a +/- sign when they printed the question. Or, it is possible that you transposed a +/- when you copied the problem from its source? Either way, you need the + sign instead of the - sign to get the answer that they claim is correct.

Does all this make sense? Let me know if you have any further questions.

Mike