Answer:
12 cm
Explanation:
The square of the length of the tangent segment is equal to the product of near and far distances to the circle from the point of intersection of the secant and tangent:
(8 cm)^2 = (4 cm)(4 cm +x)
16 cm = 4 cm +x . . . . . . divide by 4 cm
12 cm = x . . . . . . . . . . . . subtract 4 cm