The roots of polynomial $ p(z) $ are:
$$ \begin{aligned}z_1 &= 4\\[1 em]z_2 &= 2 \end{aligned} $$Step 1:
Get rid of fractions by multipling equation by $ \color{blue}{ 8 } $.
$$ \begin{aligned} 1-\frac{3}{4}z+\frac{1}{8}z^2 & = 0 ~~~ / \cdot \color{blue}{ 8 } \\[1 em] 8-6z+z^2 & = 0 \end{aligned} $$Step 2:
Write polynomial in descending order
$$ \begin{aligned} 8-6z+z^2 & = 0\\[1 em] z^2-6z+8 & = 0 \end{aligned} $$Step 3:
The solutions of $ z^2-6z+8 = 0 $ are: $ z = 2 ~ \text{and} ~ z = 4$.
You can use step-by-step quadratic equation solver to see a detailed explanation on how to solve this quadratic equation.