A flywheel slows from $543$ rev$/$min to $397$ rev$/$min while rotating through $23$ revolutions.

$($a$)$

What is the angular acceleration $($in rad$/$s$2)$ of the flywheel? $($Assume the flywheel is rotating in the positive direction. Indicate the direction with the sign of your answer.$)$

$($b$)$

How much time elapses $($in s$)$ during the $23$ revolutions?