3. \( 3 x^{2}-9 y^{2} \)

Did you know that the expression \( 3x^{2} - 9y^{2} \) is actually a difference of squares? By factoring it, you can rewrite it as \( 3(x^{2} - 3y^{2}) \), which highlights the relationship between \( x \) and \( y \). This gives you a glimpse into how algebraic identities can simplify complex expressions! Now, if you're looking to apply this in real life, imagine you're designing a rectangular garden where the length is 3 times the width. This expression can help you determine the optimal dimensions based on the area represented. So, not only do you get a math challenge, but you also get a chance to beautify your surroundings!