We know that segments AS and SD, and CR and DR are congruent and will be 3 because of the inscribed quadrilateral theorem. That means that AT and TB are both x, as well as CU and UB. Now we can set an equation.
x + x + x + 3 + 3 + 3 + 3 + x = 18
Solve
4x + 12 = 18
4x = 6
x = 1.5
