In other words, if $t(x_1) = t(x_2)$, then $x_1$ must equal $x_2$.
Let's assume $t(x_1) = t(x_2)$ and see if this implies $x_1 = x_2$:
$t(x_1) = t(x_2)$
$x_1^2 + 3 = x_2^2 + 3$
Subtracting 3 from both sides:
$x_1^2 = x_2^2$
Taking the square root of both
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