Exercise 12 is still due at 7AM on January 30'th, but it is now optional, and will count only as a bonus. In other words, if you do not submit exercise 12, then your exercise grade will equal the average of your grades on exercises 1 through 11. If you do submit exercise 12, it will count as a bonus, at half your grade on this exercise, up to 5 points (if you get the maximum grade of 10). Of course, the maximum final homework grade is 10/10 even with the bonus.
For example, if in exercises 1 through 11 you got a total of 93 points out of the possible 110 (counting each exercise as 10 points), then since 93/110 = 0.845, your average grade is 8.45 out of 10. If you then get a 9/10 on exercise 12, then you will get 4.5 bonus points. Now, since (93+4.5)/110=0.886 , your final homework grade will be 8.86 out of 10.
Whether or not you do this exercise, remember that it covers material
that may appear in the exam.