(a) From newton's second law , the rate of change of momentum of a body is directly proportional to the resultant force acting on it and is in the same direction as the resultant force .
(b) I am not familiar with the conversion of units here . You can check it from your calculator .
The change in momentum is ,
where m is given , v and u are also given so just substitute appropriately but you have to make sure the units are fixed .
The average force on the bat would be what you got in (1) divided by time (0.001)