Answer
$$2x\sqrt{x-1}$$
Work Step by Step
In order to simplify the expression, we first simplify each of the terms, and then we combine like terms. Doing this, we find:
$$ \sqrt{x^3-x^2}+\sqrt{9x^3-9x^2}-\sqrt{4x^3-4x^2} \\ x\sqrt{x-1}+3x\sqrt{x-1}-2x\sqrt{x-1}\\ 2x\sqrt{x-1}$$
