1. **State the problem:** Simplify the expression $[-3]x 2x$. 2. **Understand the expression:** The expression can be interpreted as multiplication of $-3x$ and $2x$. 3. **Write the multiplication:** $$(-3x) \times (2x)$$ 4. **Multiply the coefficients:** $$-3 \times 2 = -6$$ 5. **Multiply the variables:** $$x \times x = x^2$$ 6. **Combine the results:** $$-6x^2$$ 7. **Final answer:** The simplified expression is $-6x^2$.