Angle QRT is supplementary to angle SRT, so they add up to 180, so angle SRT's measure is 180-9x
From there you can add up all three angles inside the triangle and set them equal to 180
so [tex](180-9x) +[/tex] [tex](9+x)+5x=180[/tex]
Combine like terms and solve for x ⇒ x=3
