Your answer would be 18.39
And here's why:
Behind the decimal your numbers are in the following places.
The (3) is in the tenths place.
The (8) is in the hundredths place.
And the (6) is in the thousandths place.
Whatever number is AFTER the place your rounding, is going to help you out.
If the number is 1-4, you round down.
If the number is 5-9, you round up.
Hope this helps! (: