Question

I need help finding the radius.

Answer 1

Since the central angle is a right angle, the labeled arc is 1/4 of the total circumference of the circle. The circumference of a circle with radius $r$ is $2\pi r$, so we have

$\dfrac{2\pi r}4=18\pi\,\mathrm{in}\implies r=36\,\mathrm{in}$