Nice solution, Galactus!
By the way, these "Quickie" solutions are not meant to be "better" than yours.
Most of them employ a "trick" of some sort ... some well-known, others obscure.
I found each to be an eye-opening experience.
We are expected to know: .
. . and that: .If , then: .
So we have: .
. . Hence: .
It concludes with: "Clearly,
. . The negative square root gives a negative value for "