1. **State the problem:** Simplify the expression $21x + 14$. 2. **Identify the common factor:** Both terms $21x$ and $14$ have a common factor of $7$. 3. **Factor out the common factor:** $$21x + 14 = 7 \times 3x + 7 \times 2 = 7(3x + 2)$$ 4. **Explain:** Factoring means expressing the expression as a product of its factors. Here, $7$ is factored out because it divides both terms evenly. 5. **Final answer:** $$21x + 14 = 7(3x + 2)$$