Its a 30 60 90 triangle so the ratio
is 1, radical 3, 2
so A=9
B=radical 3 *9
sources, already took trigonometry and got 105%
Edit, looked at your work, since you already got 18 and 9 you can do 18Squared - 81 or {(9squared)}
also: cos(30)= radical 3 / 2 so 18/2=9, 9* radical 3