## Elementary Algebra

$(2a-9b)(2a+9b)$
The given expression can be written as: $=(2a)^2-(9b)^2$ RECALL: A difference of two squares can be factored using the formula: $x^2-y^2=(x-y)(x+y)$ Use the formula above with $x=2a$ and $y=9b$ to obtain: $= (2a-9b)(2a+9b)$