# How do you multiply  (sqrt6 +6c)(sqrt6 -6c) ?

You can recognise this as being of the form $\left(a + b\right) \left(a - b\right) = {a}^{2} - {b}^{2}$
$\left(\sqrt{6} + 6 c\right) \left(\sqrt{6} - 6 c\right) = {\left(\sqrt{6}\right)}^{2} - {\left(6 c\right)}^{2} = 6 - 36 {c}^{2}$