An electron moves in a circular path with a speed of $1\mathrm{.}45{10}^7\frac{m}{s}$ in the presence of a uniform magnetic field with a magnitude of $2\mathrm{.}12$ mT $.$ The electron's path is perpendicular to the field.

$($a$)$ What is the radius $($in cm $)$ of the circular path?

$$ cm

$($b$)$ How long $($in s $)$ does it take the electron to complete one revolution?

$$ s