You can start off by understanding this is not a right angle (exactly 90 degrees, think of a corner of a room) or and obtuse angle (more than 90 degrees, bigger than a right angle). With that information we know that it is an acute angle (less than 90 degrees, smaller than a right angle). With that we have 40 and 50 degrees left. When you compare the angle with the 40 degrees one, it is the same size. The angle 1 is a reflection of the triangle with the 40 degrees angle. Hope this helps :)
