The total cost includes the setup cost + per hour cost. 
Let, x represents the time and y represents the total cost. 
 .... (1)
 .... (1)
Let Y represent the cost for B's DJ service. 
 .... (2)
 .... (2) 
a. Graphing both the equations give the intersection point (solution) as (4,500) 
It means for 4 hours, the total cost of both the DJ's is same at $500. 
b. Substitution Method. 
Plug y = 125x in equation (1) 





Hence, the total cost for 4 hours in both the service is $500.
c. Addition method.
Multiply equation (2) by -1 
 .... Equation (3).
.... Equation (3).
Add euation (1) and equation (3) 




Solution: For x = 4, y = 500 ( same as the previous two methods) 
For x = 2, equation (1) 

For x = 2, equation (2)

Hence, for 2 hours, DJ B would be a better choice since it would charge $250. 
For x = 6 hours, equation (1) 

For x= 6, equation (2) 

For 6 hours, DJ A would be  better option since it charges $650 for 6 hours.