Please read and consider the difference between the two sentences below.
1) Rapunzel さんはかみが長いです。
When you learned sentence 1 (～は～がadjectiveです）in basic level, you might have wondered the difference between sentence 1 and 2.
In sentence 1, the topic is ‘Rapunzel さん’.
So the nuance of sentence 1 is that you are talking about Rupunzelさん.
Other people (for instance, her friends) might have long hair, but now you want to describe that Rupunzel has long hair.
The topic of sentence 2 is ’Rapunzelさんのかみ’.
Rupunzel might have nice eyes, or nose, but now you would like to talk about her かみ.
Here is a further explanation of the nuance of the 2 sentences:
1.「Rapunzelさん」はかみが長いです。(Compared with other girls)
2.「Rapunzelさんのかみ」は長いです。(She might have nice eyes, but you want to describe about her hair now)
Many students do not understand the clear difference between「は」and 「が」. It's very important to ask your teacher whenever you are unsure of whether to use 「は」or 「が」. The more you ask, the deeper the understanding of the difference.