1. **State the problem:** Solve the equation $X + 4 = 21$ for $X$. 2. **Formula and rule:** To isolate $X$, subtract 4 from both sides of the equation. This uses the subtraction property of equality which states if you subtract the same number from both sides, the equality holds. 3. **Apply the subtraction:** $$X + 4 - 4 = 21 - 4$$ 4. **Simplify both sides:** $$X + \cancel{4} - \cancel{4} = 21 - 4$$ $$X = 17$$ 5. **Answer:** The solution to the equation is $X = 17$. This means when $X$ is 17, the original equation $X + 4 = 21$ is true.