You can put this solution on YOUR website! The future value of a payment is:
FUTURE VALUE OF A PAYMENT
FV(PMT)=PMT*((1+i)^n -1)/i
FV = Future Value
PMT = Payment per time period
i = Interest Rate per Time Period
n = Number of Time Periods
So, here we have:
FV=4200*((1+.09)^15 -1)/.09=$123,315.84