this is an interesting problem regarding the GMAT Club Math Book. I studied it and on page 15 it says:
When the GMAT provides the square root sign for an even root, the only accepted answer is the positive root. That is, NOT +5 or -5. In contrast, the equation x^2 has TWO solutions, +5 and -5. Even roots have only a positive value on the GMAT.
Due to this fact (which of course does not include this particular situation, because it's talking about the "sign"), I didn’t consider the negative root.
Maybe a phrase, which could be improved on the next issue