What year is that in English years mate? This is simple level maths in our school, and taught from year 7. The letter is simply a substitution for a number, so just treat it like a normal equation and replace the letters with the numbers given.
You been told about Mymaths yet? Your school should have a login for it, and you can use it to help with homeworks. If you haven't been given a login, ask about it in school as it is a great learning aid both in home and in school.
James