Angle QRT is an exterior angle to the triangle. The rule is that the exterior angle of a triangle is equal to the sum of its remote interior angles. Those remote interior angles are angle RTS and angle TSR. So our equation is
. We can remove the parenthesis and simplify by combining like terms to get 9x = 5x + 9 + x and 9x = 6x + 9 and 3x = 9. That means that x = 3.