The root of polynomial $ p(x) $ is:
$$ \begin{aligned}x &= \dfrac{ 729 }{ 800 } \end{aligned} $$Step 1:
Get rid of fractions by multipling equation by $ \color{blue}{ 100 } $.
$$ \begin{aligned} -8x+\frac{729}{100} & = 0 ~~~ / \cdot \color{blue}{ 100 } \\[1 em] -800x+729 & = 0 \end{aligned} $$Step 2:
$$ \begin{aligned} -800x+729 & = 0 \\[1 em] -800 \cdot x & = -729 \\[1 em] x & = - \frac{ 729 }{ -800 } \\[1 em] x & = \frac{ 729 }{ 800 } \end{aligned} $$