# How do you factor r^2-9t^2?

$\left(r + 3 t\right) \left(r - 3 t\right)$
The problem is a difference of squares. In general, ${a}^{2} - {b}^{2}$ factors to $\left(a + b\right) \left(a - b\right)$, in this case $a$ is $r$ and $b$ is $3 t$.