**Is the following proof valid? If not, what is the invalid step in the proof?**
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:- | -: | :-
|| $~~~~(2x^2-18)(x+3)(x-3)$
Step $1$ | | $=2(x^2-9)(x^2-3x+3x-9)$
Step $2$ | | $=2(x^2-9)(x^2-9)$
Step $3$ | | $=2(x^2-9)^2$
Step $4$ | | $=2(x^4-81)$
Step $5$ | | $=2x^4-162$
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