absolute values

so you should DEFINITELY know the following:

IMPORTANT PIECE OF KNOWLEDGE:
√(x^2) is equal to |x|.

PUT THIS ON A FLASH CARD if you don't know it.

in other words, if you square a number and then take the square root of that quantity, you don't necessarily get the original number back; instead, you get the absolute value of the original number.

here's why: when you square the number, you get a positive value no matter what. in other words, the squared version - which is automatically positive - doesn't "remember" whether the original number was positive or negative. then, when you take the square root, you wind up with a number that's still positive, regardless of the sign of the number you started with.
that's the exact definition of absolute value.

examples / illustrations:
(-5)^2 is 25. take the square root and you get +5, which is the absolute value of -5.
5^2 is also 25. take the square root and you get +5, which is the absolute value of 5 as well.

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btw, what's the source of this question? it seems as though the gmat folks would put a little more substance into the questions; if you know the takeaway above (which you should - remember, flash cards are your friends), then this problem's solution is immediate.

if this is indeed an official problem, i'm rather surprised.