Answer:
$13,52.60
Explanation:
The formula to apply is

where
A= amount of money at the end
P=the amount of money to invest, principal
r=rate of interest in decimal
n=number of compoundings per year
t=time in years
Given that;
t=4 years
A=$18000
P=?
r=0.08
n=2
Substitute values in the formula

Divide both sides by 1.36856905041 to remain with P

P=$13152.56