Explanation:
if XY is a tangent, then it would be at a right angle (90°) with the radius or diameter of the circle (WX = 2×8.5 = 17).
and that means the Pythagoras equation must apply :
c² = a² + b²
with c being the Hypotenuse (the side opposite of the 90° angle).
20² = 17² + 5²
400 = 289 + 25 = 314
no, that is not a true equation.
therefore, XY and WX are not at a right angle to each other, and therefore XY is not a tangent.