1. **State the problem:** Solve the equation $w - 21 = 45$ for $w$. 2. **Formula and rule:** To isolate $w$, add 21 to both sides of the equation. This uses the addition property of equality which states if you add the same number to both sides, the equality holds. 3. **Intermediate work:** $$w - 21 + 21 = 45 + 21$$ $$w = 66$$ 4. **Explanation:** We added 21 to both sides to cancel out the $-21$ on the left side, leaving $w$ alone. On the right side, $45 + 21$ equals 66. 5. **Final answer:** $w = 66$