Two angles are called complementary when their sum is 90º. In the figure, the α and β angles together form a right angle. Two angles are called supplementary when their sum is 180º. In the figure, the α and β angles together form a straight angle.
The answer is 4 units. The distance between the two points of B and C is equals 6-4=2, because the y is the same. According to the question, the BC is equals 0.5*B'C'. So the length of B'C' is 2/0.5=4 units.
