$I={{I}_{m}}\sin \frac{2\pi }{T}t\Rightarrow I=4\sin \frac{2\pi }{0/02}t\Rightarrow I=4\sin 100\pi t$ $t=\frac{1}{600}s\Rightarrow I=4\sin 100\pi \frac{1}{600}=4\sin \frac{\pi }{6}=4\times \frac{1}{2}=2$ $\varepsilon =RI=3\times 2=6V$