Hi,
In General
A = Accumulated Amount Double
P= principal = 2800
r= annual rate = .08
n= periods per year = 4
t= years =
long does it take a $2800 investment to double if it's invested at 8% compounded quarterly
ln2= 4tln(1.02)
ln2/4ln(1.02) = t = 8.75yrs