A sinusoidal electromagnetic wave emitted by a mobile phone has a wavelength of $35\mathrm{.}4$ cm and an electric$-$field amplitude of

$5\mathrm{.}40$

$$

$10$

$$

$2$

$$

V

$/$

m

at a distance of $250$ m from the phone. Calculate $($a$)$the frequency of the wave; $($b$)$ the magnetic$-$field amplitude; $($c$)$ the intensity of the wave.