The equation you have provided is correct:
A(t)=P(1+r/n)^nt
Where P = principal amount
r =
interest rate
n =
number of times the amount is compounded in a year
t = time
in years
The value of n is 12 since it is compounded monthly.
Substituting the amount to the formula:
A(t) = $300 ( 1 + 4%/12)^(12)(8)
A(t) = $300 (1 + 0.003333)^96
A(t) = $300 (1.376351)
A(t) = $412.91
