In each sentence, if the italicized verb does not agree with the subject, circle the verb and write it correct form.

I will capitalize the verb since I don't know how to italicized.

1.Big Ma and Cassie SHARES the same bedroom.

How would I solve this?

"shares" is in the 3rd person singular present tense form. Therefore, you'd need to have a singular subject to match it.

But you don't. You have a plural subject: "Big Ma and Cassie" (two people = plural).

http://grammar.ccc.commnet.edu/grammar/sv_agr.htm

The subject is plural, but SHARES is a singular verb.

Think of it this way:

He SHARES . . .
They SHARE . . .

This site may also help you.

http://www.scientificpsychic.com/grammar/regular.html

So is the correct form of SHARES

So is the correct form on SHARES in this sentences = share?

Yes. In this sentence you must use SHARE.

Just to make sure I understand, could you please review this two too? Thanks

Most of the trees HAVE Xs marked on them.
-The subject and verb agree.

2. Cassie and her brothers ATTACKS the lumbermen.
-Attacks become attack?

Right on both of them!