In other words, we need to make sure that if $h(x_1) = h(x_2)$, then $x_1 = x_2$.
Let's assume $h(x_1) = h(x_2)$:
$x_1^2 - 4 = x_2^2 - 4$
Adding 4 to both sides, we get:
$x_1^2 = x_2^2$
Taking the square root of both sides:
$x_1 = \pm x_2$
Since $x \geq 0$,
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