The underlying formula that you want to use to solve here is
P = A / (1 + RT), where
P = principal amount you need to deposit
A = future value of the account = 6000
R = annual interest rate = 0.06
T = time in years = 15
For this set of information that means taking
P = 6000 / (1 + 0.06*15) = 3157.89
meaning that you need to deposit $3157.89 now