Answer:
b = 18
Explanation:
From an external point, the products of distances to the near circle intercept and the far circle intercept are the same. For a tangent, such as AC, point A is both the near and far intercept point, so that product is the square of the length of AC.
(AC)² = (CG)(CV)
b² = 12·27 = 324 . . . . substitute known values
b = √324 . . . . . . . . . . take the square root
b = 18