Hey can you grab those points?

How do you solve a question like this?

Credit card a offers an introductory apr of 3.4% for the first three months and standard apr of 15.7% thereafter,

Sam opened a money-market account that pays 3% simple interest. he started the account with $7,000 and made no further deposits. when he closed the account, he had earned $420 in interest. how long did he keep his account open?