1. **Stating the problem:** Find a question similar to question 3c, which is to expand and simplify an expression involving square roots: $ (\sqrt{a} + \sqrt{b})(\sqrt{a} - \sqrt{b}) $. 2. **Similar question:** Expand and simplify $ (\sqrt{x} + \sqrt{y})(\sqrt{x} - \sqrt{y}) $. 3. **Formula and rules:** Use the difference of squares formula: $$ (p + q)(p - q) = p^2 - q^2 $$ where $p = \sqrt{x}$ and $q = \sqrt{y}$. 4. **Step-by-step solution:** - Apply the formula: $$ (\sqrt{x} + \sqrt{y})(\sqrt{x} - \sqrt{y}) = (\sqrt{x})^2 - (\sqrt{y})^2 $$ - Simplify the squares of square roots: $$ = x - y $$ 5. **Explanation:** Multiplying conjugates like $ (\sqrt{x} + \sqrt{y})(\sqrt{x} - \sqrt{y}) $ eliminates the square roots because the middle terms cancel out, leaving the difference of the squares of the terms. **Final answer:** $$ x - y $$