TeX source:
\begin{array}{rclcl} 12\left(\left(\dfrac{p}{2}-\dfrac{1}{2}\right)^2-\dfrac{5}{4}p^2\right)+12p^2 &=& 9 \cr\cr 12\left(\dfrac{1}{4}p^2-\dfrac{1}{2}p+\dfrac{1}{4}-\dfrac{5}{4}p^2\right)+12p^2 &=& 9 \cr\cr 12\left(-p^2-\dfrac{1}{2}p+\dfrac{1}{4}\right)+12p^2 &=& 9 \cr\cr -12p^2-6p+3+12p^2 &=& 9 & \vert & -3 \cr -6p &=& 6 & \vert & :(-6) \cr p &=& -1 \cr\cr \mathbb{L} &=& \{-1\} \end{array}