Answer
$a = 3.00\times 10^{-2}~N/m\cdot C$
Work Step by Step
We can see that the graph changes from increasing to decreasing at $r = 2.00~cm$. This tells us that the radius of the circular region with the magnetic field is $r = 2.00~cm$
We can find $a$:
$2\pi~r~E = \vert \frac{d\Phi}{dt} \vert$
$2\pi~r~E = (\pi~r^2) ~\frac{dB}{dt}$
$2\pi~r~E = (\pi~r^2) ~(a)$
$a = \frac{2~E}{r}$
$a = \frac{(2)~(300\times 10^{-6}~N/C)}{0.0200~m}$
$a = 3.00\times 10^{-2}~N/m\cdot C$
