Answer:
a)
Price               $24  $21  $18  $15  $12   $9   $6   $3
Quantity Demanded    1     2    3    4    5    6    7    8
Marginal Revenue    $24  $18  $12  $6   $0   -$6  -$12  -$18
b) marginal revenue turns negative at 6 units
c) 5 units, since profits are maximized when MC = MR
d) $12, again profits are maximized when MC = MR