Please answer this question??

Dear Student,
According to Faraday's law, induced emf is
$$\begin{gathered} \frac{d\varphi }{dt}=\frac{d\left(a{t}^2\right)}{dt}=2at \\ As per question, \\ {\left(2at\right)}_{at t= 3s}=9 \\ So, \\ 2\times a\times 3=9 \\ a=\frac{9}{2\times 3}=\frac{3}{2}=1,5 Wb s^{-2} \end{gathered}$$

Regards