# How do you factor 49a^2 - 81b^2?

You may notice that $49 = {7}^{2} \mathmr{and} 81 = {9}^{2}$ are both squares.
So $49 {a}^{2} = {\left(7 a\right)}^{2} \mathmr{and} 81 {b}^{2} = {\left(9 b\right)}^{2}$ are also squares, and then we can use the special product of the form: ${x}^{2} - {y}^{2} = \left(x + y\right) \left(x - y\right)$
$= \left(7 a + 9 b\right) \left(7 a - 9 b\right)$