Answer
$-3x(x+1)(x-4)$
Work Step by Step
Use: $-3x=-4x+x$
$9x^2+12x-3x^3=-3x(x^2-3x-4)=-3x(x^2-4x+x-4)=-3x[x(x-4)+1(x-4)]=-3x(x+1)(x-4)$
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