Hey there!
To start, this is an exponential, meaning your equation should be in this format:
y=a(1±b)^x where a is the principal amount, b is the rate, and x is the years passed. If b is a decrease rate, you would subtract it from one, and if it is an increase rate, you would add it to 1. In this case, the rate 5.5% is a decrease rate so you would perform 1-5.5% or 1-0.055 which would be 0.945.
Now, plug in your known values:
y=3900(0.945)^15
Finally, simplify to get:
y=1669.33
Now, round to the nearest whole number (you can't have a decimal of a tree) to get 1669 trees after 15 years.
