Finding Radii of Concentric Circles with Tangent Chords

Two circles share the same centre. Chords $AB$ and $AC$ of the larger circle are tangent to the smaller circle. The angle $\angle BAC$ measures $60^\circ$.
a) Determine the radius of the larger circle given that the radius of the smaller circle is $4 \, \text{cm}$.
b) Determine the radius of the smaller circle given that the radius of the larger circle is $6 \, \text{cm}$.