[tex] a [/tex] would equal 140°.
If we draw another line parallel to both CD and AB coming from E and going to another point we can call F, we form two sets of co-interior angles, or 'C' angles, which means we can use the rule that co-interior angles add to 180°.
180 - 128 = 52°, and so because angle AEC is 92° in total, we can subtract 52 from 92 to get angle AEX = 40°. Then we use the rule again and do 180 - 40 = 140°, which is angle EAB that has been labeled [tex] a [/tex].
I hope this helps!
