we use 2 different rules here:
1) sum of angles in a triangle is equal to 180 degrees
2) when a triangle side is a diameter of the circle, then angle opposite to diameter is equal to 90 degrees.
so from 2) we know that B= 90
then from first one:
sum of angles = 180 = 90 + (3x + 6) + x = 96 + 4x
so 180 - 96 = 4x
so x = 84/4
x = 21
so answer is
A= 3x+ 6 = 63 + 6 = 69
B = 90
C = 21
