1. **State the problem:** Solve the equation $X + 13 + y = 0$ for one variable in terms of the other. 2. **Rewrite the equation:** The equation is $X + 13 + y = 0$. 3. **Isolate one variable:** To express $y$ in terms of $X$, subtract $X$ and 13 from both sides: $$y = -X - 13$$ 4. **Interpretation:** This means for any value of $X$, $y$ is the negative of $X$ minus 13. 5. **Alternatively, isolate $X$:** $$X = -y - 13$$ This gives the relationship between $X$ and $y$. **Final answer:** $$y = -X - 13$$