A dentist's drill starts from rest. After $4\mathrm{.}70$ s of constant angular acceleration it turns at a rate of $2\mathrm{.}85104$ rev$/$min$.$

$($a$)$ Find the drill's angular acceleration.

rad$/$s$2$

$($b$)$ Determine the angle $($in radians$)$ through which the drill rotates during this period.

rad