1. **State the problem:** Simplify the expression $$-12x^2 - 6x^2$$. 2. **Recall the rule:** When combining like terms, add or subtract the coefficients of terms with the same variable and exponent. 3. **Identify like terms:** Both terms are like terms because they have the same variable $x$ raised to the power 2. 4. **Combine coefficients:** $$-12 - 6 = -18$$. 5. **Write the simplified expression:** $$-18x^2$$. **Final answer:** $$-18x^2$$.