Answer:
The system of equations 3x + 7y = 80 and 6x + 3y = 63.75 can be used to determine the cost of one CD and one DVD.
Explanation:
Given that:
Cost of 3 CDs and 7 DVDs = $80
Cost of 6 CDs and 3 DVDs = $63.75
Let, 
x be the cost of one CD 
y be the cost of one DVD 
According to given statement;
3x + 7y = 80 
6x + 3y = 63.75
Hence, 
The system of equations 3x + 7y = 80 and 6x + 3y = 63.75 can be used to determine the cost of one CD and one DVD.