Answer
$x=\left\{ -\dfrac{5}{9},\dfrac{5}{9} \right\}$
Work Step by Step
Using the properties of equality, the given expression, $
81x^2-5=20
,$ is equivalent to
\begin{array}{l}\require{cancel}
81x^2-5-20=0
\\\\
81x^2-25=0
.\end{array}
Factoring the above equation, $
81x^2-25=0
,$ results to
\begin{array}{l}\require{cancel}
(9x+5)(9x-5)=0
.\end{array}
Equating each factor to zero (Zero Product Principle), then the solutions to the equation, $
(9x+5)(9x-5)=0
,$ are
\begin{array}{l}\require{cancel}
9x+5=0
\\\\
9x=-5
\\\\
x=-\dfrac{5}{9}
,\\\\\text{OR}\\\\
9x-5=0
\\\\
9x=5
\\\\
x=\dfrac{5}{9}
.\end{array}
Hence, $
x=\left\{ -\dfrac{5}{9},\dfrac{5}{9} \right\}
.$
