You could use either a different variable but in general I would use a * or a dot. Even if x isn’t a variable, people might mistaken it to be and xPLEASE don't use "x" to mean both a variable and multiplication! I can't read those first few lines. I don't think you factored the same thing out of both terms; the GCF includes an exponent of 3, which you lost.
But the important thing is that you must not distribute (expand) everything. There was a reason we factored out the GCF at the start! (Did you not compare what I said to do with the form given for the answer? Factors are your friends, here, so you don't want to send them away.) So everything after "therefore" is going in the wrong direction; you just need to correct the part above that, and then expand only what's inside my [ ... ].