If the graphs of linear equations in a system are concurrent, having the same slope and the same y-intercept, what does that mean about the possible solution(s) of the system?

Becca wants to mix fruit juice and soda to make a punch. she can buy fruit juice for $3 a gallon and soda for $4 a gallon. if she wants to make 28 gallons of punch at a cost of $3.25 a gallon, how many gallons of fruit juice and how many gallons of soda should she buy?