Each year a certain amount of money is deposited in an account which pays an annual interest rate of r so that at the end of each year the balance in the account is multiplied by a growth factor of x=1+r. $500 is deposited at the start of the first year, an additional $200 is deposited at the start of the next year, and $600 at the start of the following year What is an expression for the value of the account at the end of three years in terms of the growth factor x?