$z=2$ is extraneous.
Work Step by Step
$-z=\sqrt -z+6$ With $z=-3$ then $-(-3)=\sqrt -(-3)+6$ $3=\sqrt 9$ $3=3$ With $z=2$ then $-(2)=\sqrt -(2)+6$ $-2=\sqrt 4$ $-2\ne 2$ $z=2$ is extraneous.
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