How do you factor completely 100 - 9x^2?

$\left(10 - 3 x\right) \left(10 + 3 x\right)$
The expression $100 - 9 {x}^{2}$ is the difference of two perfect squares and can be factored as follows:
$\left(10 - 3 x\right) \left(10 + 3 x\right)$