Rule: The inscribed quadrilateral inside the circle has the opposite angles add to 180 (aka they are supplementary)
So we'll add up angles R and T, and set that sum equal to 180 like so
R+T = 180
Replace R and T with their respective expressions in terms of x leading to this
(x+9) + (x-11) = 180
Let's solve for x
x+9 + x - 11 = 180
2x - 2 = 180 ......... combine like terms
2x-2+2 = 180+2 ... add 2 to both sides
2x = 182
2x/2 = 182/2 ..... divide both sides by 2
x = 91 which is the final answer