1. **State the problem:** Given the value $a=3$, we want to understand or use this value in a mathematical context. 2. **Explanation:** The variable $a$ is assigned the value 3. This means wherever $a$ appears in an expression or equation, it can be replaced by 3. 3. **Example usage:** If we have an expression like $y = a^2 + 2a + 1$, substituting $a=3$ gives: $$y = 3^2 + 2 \times 3 + 1$$ 4. **Simplify:** $$y = 9 + 6 + 1 = 16$$ 5. **Conclusion:** The value of $y$ when $a=3$ is 16. This demonstrates how to substitute and evaluate expressions with a given variable value.