Question: User: $\left\{x+y=9\right\}$
1. **State the problem:** Solve the equation $$x + y = 9$$ for one variable in terms of the other.
2. **Formula and rules:** This is a linear equation in two variables. To express one variable in terms of the other, isolate one variable on one side.
3. **Isolate $y$:**
$$y = 9 - x$$
4. **Explanation:** We subtract $x$ from both sides to isolate $y$. This shows that for any value of $x$, $y$ is $9$ minus that value.
5. **Alternative:** Similarly, we can isolate $x$:
$$x = 9 - y$$
6. **Summary:** The equation represents all pairs $(x,y)$ such that their sum is $9$.