TeX source:
\begin{array}{rclcl} \mathbb{D} &=& \mathbb{R} \cr\cr \dfrac{9}{4}\left(\dfrac{4x}{3}\right)^4-4x\left(\dfrac{4x}{3}\right)^3 &=& -192 \cr\cr \dfrac{9}{4} \cdot \dfrac{256}{81}x^4-4x \cdot \dfrac{64x^3}{27} &=& -192 \cr\cr \dfrac{64}{9}x^4-\dfrac{256}{27}x^4 &=& -192 \cr\cr -\dfrac{64}{27}x^4 &=& -192 & \vert & :\left(-\dfrac{64}{27}\right) \cr\cr x^4 &=& 81 & \vert & \pm\sqrt[4]{} \cr x_{1,2} &=& \pm3 \cr\cr \mathbb{L} &=& \{-3;3\} \end{array}