Answer
10.9 meters farther.
Work Step by Step
From the textbook, on page 240, “An object whose center of gravity above its base of support will be stable if a vertical line projected downward from the center of gravity falls within the base of support.”
From the given information, the base of support is a circle of radius 7.7 m.
The tower will be unstable if the CG is more than 7.7 meters off center. which means that the top is 2(7.7m) = 15.4 m off center. The top is currently 4.5m off center, so it would have to move 15.4 m– 4.5m = 10.9 m farther to be unstable.