Any straight line will have the measure of its angle as 180°. In the picture, ∠AED is that. So all the pieces of that angle must add up to 180 as well.
There are three smaller angles that make up that line, one that is 57°, one that is 33°, and our third piece. All three add up to 180, so,
57 + 33 + third piece = 180
90 + third piece = 180
third piece = 90
That third piece is ∠BEC, which is 90°. Any angle measuring 90° is a right angle.
So ∠BEC is a right angle.
