This is geometry, angles in inscribed quadrilaterals,

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