Hey there! The answer is opposite angles (or vertical angles).
Angles 1 and 2 are opposite angles (or vertical angles, which is a synonym), which means they are equal in size.
In this particular situation angles 1 and 2 are also perpendicular angles, which means they measure 90 degrees. Because the angle 'above angle 2' in the picture is 90 degrees, its opposite angle is also 90 degrees. Since the sum of the four angles is 360 degrees and since angles 1 and 2 are opposite angles, they must be 90 degrees and therefore perpendicular.