TeX source:
\begin{array}{rcl}f\left(\sqrt{-14.409}\right) &=& \dfrac{\sqrt{\left(\sqrt{-14.409}\right)^2-9}}{4}-3\\\\&=& \dfrac{\sqrt{14.409-9}}{4}-3\\\\&=& \dfrac{\sqrt{14.400}}{4}-3\\\\&=& \dfrac{120}{4}-3\\\\&=& 30-3\\&=& 27\\\end{array}