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Which of the following expressions below is equivalent to the expression:

$$ \frac { { x }^{ 2 }-6x+9 }{ { x }^{ 2 }-9 } $$

...where $ x\neq \pm 3 $?

$ -1 $

$ -6x $

$ \cfrac { 2 }{ 3 } x-1 $

$ -\cfrac { 3 }{ x-3 } $

$ \cfrac { x-3 }{ x+3 } $