1. **State the problem:** Factorise the expression $9 - 4n^2$. 2. **Recognize the formula:** This is a difference of squares, which follows the rule: $$a^2 - b^2 = (a - b)(a + b)$$ 3. **Identify $a$ and $b$:** Here, $9 = 3^2$ and $4n^2 = (2n)^2$. 4. **Apply the difference of squares formula:** $$9 - 4n^2 = 3^2 - (2n)^2 = (3 - 2n)(3 + 2n)$$ 5. **Final answer:** The factorised form is $$\boxed{(3 - 2n)(3 + 2n)}$$