I used to write my answer in image, But when I want to post it, it said need 10 reputations
How do I must write?
To be clear, "MathJax" is a version of "TeX" or "LaTeX", which is the current professional standard for typesetting mathematics. It's a little goofy, but not so hard to learn for writing basic mathematical things. In any case, if you are wanting to pursue mathematics, being able to write/typeset things in this way is an absolute essential.
In particular, on MathStackExchange, and elsewhere, people do not like "images", for several reasons. First, they involve external links, which are an un-necessary danger. Second, the link can get broken. Third, often the image is hard to read (especially for people with visual difficulties).
Also, screen-shot+paste-image is a reeeeeally low-effort way to ask a question, and the general style here is to demonstrate some self-effort at addressing the question you want to ask. An image is rather negative evidence of effort. :)