The roots of polynomial $ p(y) $ are:
$$ \begin{aligned}y_1 &= 17\\[1 em]y_2 &= 12 \end{aligned} $$The solutions of $ y^2-29y+204 = 0 $ are: $ y = 12 ~ \text{and} ~ y = 17$.
You can use step-by-step quadratic equation solver to see a detailed explanation on how to solve this quadratic equation.