TeX source:
\begin{array}{rclll} f(x) &=& \sqrt{-6x^2+16x+1} \cr \cr f\left(\dfrac{4}{3}\right) &=& \sqrt{-6 \cdot \left(\dfrac{4}{3}\right)^2+16 \cdot \dfrac{4}{3}+1} &=& \dfrac{\sqrt{105}}{3} \end{array}