In the first circle, x is the radius and also the hypotenuse of a right triangle with legs measuring 9 and 12. So by the Pythagorean theorem,
x² = 9² + 12² = 225 = 15² ⇒ x = 15
In the second circle, the chords AB and CD have the same length, and each of AP, BP, CP, and DP are radii of the circle and also have the same length. This means ∆ABP and ∆CDP are congruent, so the two labeled angles are also congruent, so x = 54.