Hello, AHDDM!

We are given: and

Let , then

is inscribed in a semicircle.

. . Hence, is a right triangle.

is the altitude to the hypotenuse of a right triangle.

Theorem: The altitude to the hypotenuse of a right triangle is

. . the mean proportional of the two segments of the hypotenuse.

Hence: .

We have the quadratic: .

. . which factors: .

. . and has the positive root: .

Therefore: . cm.