It is discovered that the college registration process for Professor Smith's Calculus class can be modeled by the equation y=2x+12, where y = the number of students registered at the end of each day, and x = the day of registration. Professor Smith has 12 students pre-registered before the registration process begins, and the registration closed after 7 days.

While it can be said that y=2x+12 models the registration process for this Calculus class, you can see that the "real" model is only a portion of this straight line. There are no "negative" days for the registration process, and there are no days beyond Day 7 for registration.