Could I get some help with this math problem? I'm a complete idiot when it comes to math.

The cost of water use (in dollars) is a linear function of the amount of water used (in hundreds of cubic feet, HCF), as shown in the figure below. The cost for using 14 HCF of water is $39.40, and the cost for using 28 HCF is $63.90. What is the cost for using 19 HCF of water?

No clue on how to solve this.

Please help.