I assume that the segment that measures 13.5 cm is a tangent to the circle at the point we see. The side that measures x is a radius of the circle, so the side of the triangle that measures x and the side that measures 13.5 cm are perpendicular. The triangle is a right triangle. One leg measures x. One leg measures 13.5 cm. The hypotenuse measures x + 8.45 cm. With the lengths of the the 3 sides, we can use the Pythagorean theorem and solve for x.