How do you make this into a parrell sentence? Thank you

Palm readers examine lines on the palm, fingers and their lengths, and the prominence of the mounts.

I left an important word out!

" ... includes a noun, but they are not parallel."

To make the sentence into a parallel structure, you would need to ensure that the items being listed are in the same grammatical form. In this case, the sentence lists three things that palm readers examine:

1. Lines on the palm.
2. Fingers and their lengths.
3. The prominence of the mounts.

To make this parallel, you can rewrite the sentence as follows:

"Palm readers examine lines on the palm, lengths of fingers, and the prominence of the mounts."

By using the same noun phrase structure ("lines on the palm," "lengths of fingers," and "prominence of the mounts") for each item in the list, we create a parallel sentence.

Any series (three or more items) needs to have all its parts in the same structure -- all nouns; all adjectives; all nouns with one adjective; all present participles; etc.

Here's what you have:

Palm readers examine lines

on the palm,
fingers and their lengths,
prominence of the mounts

You could say that each of the items in the series includes a noun, they are not parallel. Here's why:

~~on the palm = noun in prepositional phrase
~~fingers and their lengths = 2 nouns, one with a modifier
~~prominence of the mounts = noun with a prepositional phrase (including a 2nd noun) modifying it

How can you re-write this sentence so that all elements of the series are phrased with the same construction?

Read this carefully before you decide:
http://grammar.ccc.commnet.edu/grammar/parallelism.htm