The roots of polynomial $ p(p) $ are:
$$ \begin{aligned}p_1 &= 0\\[1 em]p_2 &= 50 \end{aligned} $$Step 1:
Write polynomial in descending order
$$ \begin{aligned} 400p-8p^2 & = 0\\[1 em] -8p^2+400p & = 0 \end{aligned} $$Step 2:
Factor out $ \color{blue}{ -8p }$ from $ -8p^2+400p $ and solve two separate equations:
$$ \begin{aligned} -8p^2+400p & = 0\\[1 em] \color{blue}{ -8p }\cdot ( p-50 ) & = 0 \\[1 em] \color{blue}{ -8p = 0} ~~ \text{or} ~~ p-50 & = 0 \end{aligned} $$One solution is $ \color{blue}{ p = 0 } $. Use second equation to find the remaining roots.
Step 3:
To find the second zero, solve equation $ p-50 = 0 $
$$ \begin{aligned} p-50 & = 0 \\[1 em] p & = 50 \end{aligned} $$