1. **State the problem:** Solve the linear equation $3x + y = 9$ for $y$ in terms of $x$. 2. **Formula and rules:** To isolate $y$, subtract $3x$ from both sides of the equation. 3. **Intermediate work:** $$3x + y = 9$$ Subtract $3x$ from both sides: $$\cancel{3x} + y - \cancel{3x} = 9 - 3x$$ which simplifies to $$y = 9 - 3x$$ 4. **Explanation:** We moved the term $3x$ to the right side by subtracting it from both sides, leaving $y$ alone on the left side. This gives the equation of $y$ in terms of $x$. **Final answer:** $$y = 9 - 3x$$