Answer:
 The total amount of money Alison and Justins father donated be $108 .
Step-by-step explanation:
Distributive property.
Let a, b and c be any real numbers.
Thus
a.( b + c ) = a.b + a.c  
This is distributive property.
As given
Alison and Justins father donated $3 every lap they swam in a swim-a-thon.
Alison swam 21 laps and justin swam 15 laps.
Total amount of money Alison and Justins father donated = Cost of each lap (Number of laps of Alison father  + Number of laps of Justins father )
As 
 Cost of each lap = $3 
Number of laps of Alison father = 21 laps
 Number of laps of Justins father = 15 laps 
Putting in the above
Total amount of money Alison and Justins father donated = 3(21 + 15)
                                                                                                 = 3 × 21 + 3 × 15
                                                                                                 = 63 + 45
                                                                                                 = $ 108
Therefore the total amount of money Alison and Justins father donated be $108 .