Please help me with the first half

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