Hiya,

Both your initial answer, and the one you expected, were correct! They're the same thing written differently. But there is a much easier method.

If you start at

Now add to both sides:

Now take away from both sides:

Now take away from both sides:

So if you just reverse all of the +/- signs, you'll get the same result for the intercepts. Isn't that much easier to factorize?