What is a root? and please another my other question...thank you

A root word is one that can be expanded.

Examples: agree is a root word because we can add to it -- disagree, agreement, agreeably

The answer to your question about studying for English benchmarks is obvious as I read your last statement:

"i am having a A in english and i lived in California"

You need to work on verb tenses and capitalization. Your sentence should be:
I have an A in English and I live in California.

thank...do you have any suggestions on how to improve them?

Try this site for practice.

http://www.englishpage.com/verbpage/verbtenseintro.html

A root can have different meanings depending on the context in which it is used. Here are a couple of common definitions:

1. In mathematics, a root refers to the solution or value that satisfies an equation. For example, the equation "x^2 - 4 = 0" has two roots: +2 and -2, since plugging in those values for x would make the equation true.

2. In biology, a root is the part of a plant that typically lies below the surface of the soil and absorbs water and nutrients from the ground.

Now, please feel free to ask your other question, and I'll be happy to help you find the answer.