Step-by-step explanation:
since DE is parallel to the diameter RS, D and E are just a transformation of R and S, where R and S slid up the same distance on the circle arc.
therefore, RD = SE.
and therefore d is half of the supplementary angle to the angle DE (that means together the angle DE and the supplementary angle are 180°).
so,
d = (180 - 50)/2 = 130/2 = 65°
