Explanation:
y=42
x=90
Since the triangle has the hypotenuse as the diameter of the circle, the angle opposite the hypotenuse which is x is equal to 90 degrees.
Whenever there is a triangle with the corner of the triangle touching the side of the triangle, the distance between the two legs of the triangle on the circle is 2 times the angle of the corner that is touching. So 21(2)=42
Hope this helps :)