Answer:
Step-by-step explanation:
The tangent segment is perpendicular to the diameter, so the triangle is a right triangle (not just based on its appearance). A property of circles is: a tangent is perpendicular to the radius drawn to the point of tangency.
In the right triangle, one leg has length 12.6, and the hypotenuse has length 21.
Apply the Pythagorean Theorem: (leg)^2 + (leg)^2 = (hypotenuse)^2
Oh, and the diameter has length 2x!