Answer
$a^2-\dfrac{4}{5}a+\dfrac{4}{25}$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the given expression, $
\left( a-\dfrac{2}{5} \right)^2
$, is equivalent to
\begin{array}{l}\require{cancel}
(a)^2+2(a)\left( -\dfrac{2}{5} \right)+\left( -\dfrac{2}{5} \right)^2
\\\\=
a^2-\dfrac{4}{5}a+\dfrac{4}{25}
.\end{array}
