The "formula" for computing compound interest is given by:
A = P(1 + r /n)^(n * t)
Where A = total amount accumulated after some specified period of time
P = the amount invested (18000 in our case)
r = annual interest rate expressed as a decimal (.09 here)
n = number of compoundings per year (semiannually = 2)
t = number of years = (6)
So we have
A =
18000( 1 + (.09 / 2))^(6 * 2)
And there you are......
+128731 
