the outside line touching the circle, the 13.5 one, we don't have enough info to say is a secant, like the 8.45 one, so the 13.5 is a tangent line, which is touching the radius chord, and the point of tangency is always a right angle.
Assuming that the side of the triangle marked 13.5 is tangent to the circle, the there's a right angle where the radius meets the tangent segment. The triangle is then a right triangle and the Pythagorean Theorem applies.
