Write the equation out like x can represent the age of his goldfish so you can write x = 21 - 12 (since Boris's hamster is 12 months older than the goldfish would be 12 months younger) so it's simple 21 - 12 and you would get 9. The goldfish would be nine months old. 21 - 9 is 12 (how much older the hamster is verifying your answer. And you're no bother by the way and if you have a question about my work just ask :D!