1. **State the problem:** Solve the equation $-3a + b = 15$ for one variable in terms of the other. 2. **Choose a variable to solve for:** Let's solve for $b$. 3. **Isolate $b$:** Add $3a$ to both sides: $$-3a + b + 3a = 15 + 3a$$ which simplifies to $$b = 15 + 3a$$ 4. **Explanation:** We moved $-3a$ to the right side by adding $3a$ to both sides, isolating $b$ on the left. 5. **Final answer:** $$b = 15 + 3a$$