1. **State the problem:** Simplify the expression $-10X + (-4X)$. 2. **Recall the rule:** When adding terms with the same variable, add their coefficients. 3. **Apply the rule:** $-10X + (-4X) = (-10 + (-4))X$. 4. **Simplify the coefficients:** $-10 + (-4) = -14$. 5. **Write the final expression:** $-14X$. So, the simplified form of $-10X + (-4X)$ is $-14X$.