Answer
The magnitude of the induced emf is $~~2.3\times 10^4~V$
Work Step by Step
Note that $\frac{di}{dt}$ is the slope of the $i$ versus $t$ curve.
We can find the induced emf:
$\mathscr{E}_L = -L~\frac{di}{dt}$
$\mathscr{E}_L = -(4.6~H)~(\frac{-5.0~A}{1.0\times 10^{-3}~s})$
$\mathscr{E}_L = 2.3\times 10^4~V$
The magnitude of the induced emf is $~~2.3\times 10^4~V$.
