How do you multiply (3f - 9) ( 3f + 9)?

Feb 1, 2017

$\text{The answer is } 9 \left({f}^{2} - 9\right)$

Explanation:

$\left(a - b\right) \left(a + b\right) = {a}^{2} - {b}^{2}$

$\text{let 3f=a" , } 9 = b$

${\left(3 f\right)}^{2} - {9}^{2} = 9 {f}^{2} - 81$

$\text{or } 9 \left({f}^{2} - 9\right)$