Answer:
(c) 105°
Step-by-step explanation:
Opposite angles of an inscribed quadrilateral are supplementary.
(5x +15) +(4x +3) = 180
9x +18 = 180 . . . . . simplify
x +2 = 20 . . . . . . . divide by 9
x = 18 . . . . . . . . . . subtract 2
Angle RST is ...
5x +15 = 5(18) +15 = 105
The measure of angle RST is 105°.