$(1$ point$)$ Use part I $of$ the Fundamental Theorem $of$ Calculus $to$ find the derivative $of$

$y={\displaystyle \underset{3-6x}{\overset{5}{}}}\frac{u^5}{1+u^2}du$

$[NOTE$: Enter a function $as$ your answer. Make sure that your syntax $is$ correct, $i.e.$ remember $to$ put all the necessary $(,),$ etc.$]$