9514 1404 393
Answer:
150°
Explanation:
The rule here is that the external angle is half the difference of the arcs subtended. That relation is used to form an equation, which can then be solved for whatever measure is of interest.
(RB -EC)/2 = D
((13x +7) -(60))/2 = (5x -10)
13x -53 = 10x -20 . . . . . . . . . . multiply by 2
3x = 33 . . . . . . . . . . . . . . add 53-10x to both sides
x = 11 . . . . . . . . . . . . divide by 3
RB = 13x +7 = 13·11 +7 = 150
The measure of arc RB is 150°.