1. **State the problem:** Given the equation $21 - x = y - 5 = 15$, find the value of $x - y$. 2. **Understand the equation:** The expression $21 - x = y - 5 = 15$ means both $21 - x = 15$ and $y - 5 = 15$. 3. **Solve for $x$:** From $21 - x = 15$, subtract 15 from both sides: $$21 - x = 15$$ $$21 - 15 = x$$ $$x = 6$$ 4. **Solve for $y$:** From $y - 5 = 15$, add 5 to both sides: $$y - 5 = 15$$ $$y = 15 + 5$$ $$y = 20$$ 5. **Find $x - y$:** Substitute the values found: $$x - y = 6 - 20 = -14$$ **Final answer:** $$x - y = -14$$