First, we should find the third angle in that triangle. Since the angles form a straight line, we know they are supplementary (adding up to 180°). Let's set it up like this:
9x + R = 180
R = 180 - 9x
Now we have the value of angle R, the third angle in the triangle. Since all angles in a triangle add up to 180°, we can set it up like this:
R + S + T = 180
180 - 9x + 9 + x + 5x = 180
189 - 3x = 180
-3x = -9
x = 3
The value of x is 3°.