Setup: Let $u=13-x$.

Solve: Subtract $5$: $\dfrac{2u}{7}=3u$. Multiply by $7$: $2u=21u$, so $u=0$. Thus $13-x=0$, $x=13$, and $13+x=26$.

Answer: 26

Common slips:

• Dividing by $u$ before considering $u=0$.
• Reporting $x=13$ instead of $13+x$.
• Forgetting to subtract 5 from both sides.