Because angle ACD is 90 degrees (from the square marker symbol), this means that angle BCD is 90 degrees. The two angles add to 180 to form a straight line so 90+90 = 180. Now that we know angle BCD is 90 degrees, this means that breaking up this angle will have the sub-angles add back up to 90. Therefore, angle DCE adds to angle BCE to get 90 degrees. This is why the two angles are complementary.