The value of the exterior angle next to a triangle is equal to the sum of two angles of the triangle, opposite of the interior angle. In this case: Angle QRT is equal to the sum of angle RTS and angle RST. So if we put that into an equation it will look like this:
9x = 5x + (9 + x)
Now we can solve for x.
9x = 5x + 9 + x
9x = 6x + 9 (Combine like terms)
9x - 6x = 9 (transpose)
3x = 9
3x/3 = 9/3
x = 3
