1. **State the problem:** Simplify the expression $6x + 7x$. 2. **Formula and rules:** When adding like terms, add their coefficients and keep the variable part the same. 3. **Intermediate work:** $$6x + 7x = (6 + 7)x$$ 4. **Simplify:** $$= 13x$$ 5. **Explanation:** Since both terms have the variable $x$, we add the coefficients $6$ and $7$ to get $13$, then multiply by $x$. **Final answer:** $13x$