It is given in the question that SQ bisects angle TSR . It means SQ divide angle TSR in two equal angles which are angle RSQ and angle TSQ , and they both are equal.
Or we can say that the measurement of each angle is half of the measurement of angle TSR . So angle TSR is double of each of the angle. That is
[tex]\angle TSR \ = \ 2 (3x-9)[/tex]
And that's the required expression .
