The magnetic flux linked with a coil at any instant t is given by...

Given, $$\begin{aligned} \phi & =5 t^3-100 t+300 \\ e & =-\frac{d \phi}{d t}=\frac{-d}{d t}\left(5 t^3-100 t+300\right) \end{aligned}$$\begin{gathered} =-\left(15 t^2-100+0\right) \\ =-15 t^2+100 \\ \therefore \quad \frac{e}{t}=-15(3)^2+100=-15 \times 9+100 \\ =-135+100=-35 \mathrm{~V} \end{gathered}$ Minus sign signifies the Lenz's law