I can't figure out why the square root of this algebraic equation won't have a positive and a negative root

GRE Quant Problem Solving
1

As per my knowledge, the square root of a number always has a positive and a negative root. For example sq_root(x^2) = +x and -x.

In this question, greg arrived at C as the answer by only considering the positive root. If we consider a negative root as well, D should be the answer. Why is the root for the LHS only positive

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2

https://magoosh.com/gre/positive-and-negative-square-roots-on-the-gre/ . Summary : Symbol then consider only positive , variables : consider both +ve and -ve.