Explanation:
the sum of all angles around a single point on one side of a line is always 180°, because the line can be seen as the diameter of a circle (cutting the circle in half). and that point would be the center of the circle. one side of the line means one of the half-circles. and a half-circle stands for 180°.
so, in our case, the line is CD. the given point is O.
there are 2 angles at the top side of that line that cover together the whole side of the line. so, together they represent 180° (supplementary angles).
and so we have
180 = (8x - 11) + (2x - 9) = 8x - 11 + 2x - 9 = 10x - 20
200 = 10x
x = 20