Explanation:
every triangle inscribed into a circle with the baseline being a diameter of the circle must be a right-angled triangle.
therefore,
angle XVY = 90°.
for VY we can use Pythagoras
c² = a² + b²
with c being the Hypotenuse (the line opposite of the 90° angle, in our case XY).
XY = 2×ZY = 2×17 = 34.
so,
34² = 30² + VY²
VY² = 34² - 30² = 1156 - 900 = 256
VY = 16