Step-by-step explanation:
If Tiger is writing to his piano teacher, the letter should be formal. This means he should only use formal language as they are most likely not close friends. Formal language means no slang or improper english words. The word "cool" is usually considered slang in this context. He could use this word if it was a letter to his close friend but since it is a teacher, he must replace the word with a proper adjective, such as "brilliant" or "amazing" or any other synonyms :)