Michael started a savings account with $300. After 4 weeks, he had $350 dollars and after 9 weeks, he had $412.50. What is the rate of change of money in his savings account per week?

Can you me with this question? i'll reward *30 points i just need with this one question so that i could solve the others. * edit: i originally set up the question to reward 30 points, but for some reason i can only reward 15. if you don't get 30, then i'm sorry. i'm still kind of new to this site