Answer:
17 nights.
Step-by-step explanation:
The total length of the scarf that should be knitting = 34 cm
Sarah has already knit 17 centimeters of a scarf.
So, the remaining = 34 - 17 = 17 cm
Sarah can knit 1 centimeter each night.
So, the number of nights will Sarah have to spend knitting = 17/1 = 17 nights.