Answer:
B
Step-by-step explanation:
<STR = <SRT Given the markings of the triangle
<STR + <STU = 180 Supplementary angles
(2x - 9) + 7x = 180 Remove the brackets
2x - 9 + 7x = 180 Combine
9x - 9 = 180 Add 9 to both sides
9x = 180 + 9 Combine
9x = 189 Divide by 9
9x/9 = 189/9
x = 21
