Ok so you spent 19 dollars the difference between prices is 5 dollars and your answer would be 12 and 7. If you need any help let me know.
For this case we have that by definition, the equation of the line of the slope-intersection form is given by:

Where:
m: It is the slope of the line
b: It is the cut-off point with the y axis.
According to the data of the statement we have the following points:

We found the slope:

Thus, the equation is of the form:

We substitute one of the points and find b:

Finally, the equation is:

Answer:

2.74x7.5=20.55=20.550
That's all of your answers.