In other words, if $f(x_1) = f(x_2)$, then $x_1$ must equal $x_2$. Let's check this for the given function:
$f(x_1) = x_1 - 4$
$f(x_2) = x_2 - 4$
If $f(x_1) = f(x_2)$, then:
$x_1 - 4 = x_2 - 4$
Adding 4 to both sides, we get:
$x_1 = x_2$
Since $x_1$ must
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