Simplify the expression y = ([3]x - 2x)3 and find its derivative with respect to x.

First, simplify the term inside the cube root: $x - 2x = -x$.
Then, substitute this back into the expression for $y$:

$$y = (\sqrt[3]{-x})^3$$

The cube root and the cube power cancel each other out:

$$y = -x$$

We need to find $\frac{dy}{dx}$ for $y = -x$.
Using the power rule for differentiation, $\frac{d}{dx}(cx) = c$:

$$\frac{dy}{dx} = \frac{d}{dx}(-x)$$ $$\frac{dy}{dx} = -1$$

The derivative is:

$$-1$$