Ответ:

There are several rules of writing a formal letter.You can not use informal words like "hi" or "gonna",instead of these words ,you should use formal option "hello" and "going to".And that brings us to the first rule of writing a formal letter:using a formal words.And you should write the full name of person you are writing to.At the you can use the word"sincerely yours _____(your name)