Question

How do I find this ?

Answer 1

notice if we know the side SV = 9, then... wait a second!!!!  SV is the radius of the circle, but but but, TS is also a radius, but but but, SU is also a radius.

TU = TS + SU

TU = 9 + 9.

recall, the radius is the same length no matter how you slant it.

Answer 2

The radius $\overline{SU}=\overline{ST}=\overline{SV}$

The diameter: $\overline{TU}$

The chord $\overline{WX}$ and diameter is a chord.

$\overline{TU}=2\overline{SV}$ therefore if $|\overline{SV}|=9$

then $|\overline{TU}|=2\cdot9=18$