Answer
$m^2-\dfrac{4}{9}$
Work Step by Step
Using $(a+b)(a-b)=a^2-b^2$ or the product of the sum and difference of like terms, the given expression, $
\left( m-\dfrac{2}{3} \right)\left( m+\dfrac{2}{3} \right)
$, is equivalent to
\begin{array}{l}\require{cancel}
(m)^2-\left( \dfrac{2}{3} \right)^2
\\\\=
m^2-\dfrac{4}{9}
.\end{array}
