The formula for the value (A) of such an account with principal value P is ...
... A = P + P·r·t
where r is the interest rate and t is the number of years.
Filling in the given numbers, you have
... 35560 = 28000 + 28000·0.09·t
Subtracting 28000 and dividing by the coefficient of t, you have
... 7560/2520 = t = 3
Sara must keep the money in the account for 3 years to earn enough for a down payment.
