I saw this same question in a Mc Graw Hill book and still I do not fully understand, because Official Guide says that we must consider only the positive root.
Here is an explanation from Manhattan GMAT
:http://www.manhattanprep.com/gmat/blog/ ... -the-gmat/
Encourage cooperation! If this post was very useful, kudos are welcome
"It is our attitude at the beginning of a difficult task which, more than anything else, will affect It's successful outcome" William James