The most important bit of information here, besides the fact the AC = 6, is the fact that the exterior angle measures 150°. This means that angle ACB measures 30° (remember, a straight line has an angle measure of 180°). Since this is a right triangle, and one of the other angles measures 90°, we know that other angle measures 60° (because a triangle always has a total of 180°). This is a 30-60-90 triangle. In a 30-60-90 triangle, the ratio of the lengths of the sides is always 1::√3::2. AC, which measures 6, is the longest side. AB is the shortest side. The longest side of a 30-60-90 triangle is always twice the length of the shortest side. Half of 6 is 3. The answer is (B).
source: charlottemathtutor.com
