Concentric Circles and Tangent Length

Let $R$ be the radius of the outer circle and $r$ be the radius of the inner circle. $C$ bisects $AB$, so $AC = 12$. In the right-angled triangle formed by the center $O$, point $C$, and point $A$: $R^2 - r^2 = AC^2 = 12^2 = 144$ $(R - r)(R + r) = 144$

Since $R$ and $r$ are positive integers, testing the Pythagorean triple $(5, 12, 13)$ yields: $13^2 - 5^2 = 169 - 25 = 144$ Thus, $R = 13$ metres.