Answer: 6x - 18 (choice C)
Angle TSR is the same as angle RST. The order of the letters doesn't matter as long as the middle letter is the same (known as the vertex point).
Segment SQ bisects angle RST, so that means the two smaller pieces (angle TSQ and angle RSQ) are exactly half in measure compared to angle RST. Put another way, angle RST is twice that of angle RSQ
So,
angle RST = 2*(angle RSQ)
angle RST = 2*(3x - 9)
angle RST = 2*(3x) + 2*(-9)
angle RST = 6x - 18