Again same question, worth more points

What is the difference between the predicted value and the actual value

20.ju inces. which ordered pair is a viable solution if x represents the number of books he orders and y represents the total weight of the books, in ounces? (-3,-18) (-0.5, -3) (0,0) (0.5, 3)

Idon't get undoing if its pass adding and subtracting so can someone ? x-2 over 5 = 18