## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$9x^2-16y^2$
Recall the difference of two squares formula: $(a-b)(a+b)=a^2-b^2$ By applying this formula, we get: $$(3x + 4y)(3x - 4y)= (3x)^2-(4y)^2=9x^2-16y^2 .$$