**Is the following proof valid? If not, what is the invalid step in the proof?**
|||
:- | -: | :-
|| $~~~~x(9x+3)+3(x+3)$
Step $1$ | | $=9x^2+6x+9$
Step $2$ | | $=(3x+3)^2$
Step $3$ | | $=(3(x+1))^2$
Step $4$ | | $=9(x+1)^2$
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