Answer
5.0 s.
Work Step by Step
$$i_D=\epsilon\frac{d\Phi_E}{dt}$$
$$i_D=(3.5\times10^{-11}F/m)(24.0\times10^3 V \cdot m/s^3)t^2$$
Solve for t.
$$t =\sqrt{\frac{i_D}{(3.5\times10^{-11}F/m)(24.0\times10^3 V \cdot m/s^3)}} $$
Set $i_D=21\times10^{-6}A$.
$$t =\sqrt{\frac{21\times10^{-6}A }{(3.5\times10^{-11}F/m)(24.0\times10^3 V \cdot m/s^3)}}=5.0s $$