A grindstone increases in angular speed uniformly from $4\mathrm{.}40$ rad$/$s to $16\mathrm{.}5$ rad$/$s in $3\mathrm{.}50$ s$.$

$($a$)$ Calculate the grindstone's angular acceleration.

rad$/$s$2$

$($b$)$ Through what angle does it turn during that time?

rad