1. **Stating the problem:** Simplify the expression $3x^2 - 10x^2$. 2. **Formula and rules:** When subtracting like terms, subtract their coefficients and keep the variable part unchanged. 3. **Intermediate work:** $$3x^2 - 10x^2 = (3 - 10)x^2$$ 4. **Simplify the coefficients:** $$3 - 10 = \cancel{3} - 10 = -7$$ 5. **Final expression:** $$-7x^2$$ 6. **Explanation:** Since both terms have $x^2$, we subtract the coefficients $3$ and $10$ to get $-7$, then multiply by $x^2$. **Answer:** $-7x^2$