If you are trying to find the area of each side you created when dividing the triangle into thirds, you should divide your numbers. Since the area of the whole triangle is 18 sq. units, and you need three separate pieces from the triangle, you can divide 18 by three. This would look like this:
18 / 3 = ?
And 18 / 3 = 6. So from that you can tell that each third of the triangle is going to equal 6 sq. units.
Hope that helps!