Answer:
 4
Explanation:
The product of the secant lengths to the near and far circle intersection points is the same as the square of the tangent length.
 x(x +12) = (x +4)²
 x² +12x = x² +8x +16 . . . . eliminate parentheses
 4x = 16 . . . . . . subtract x² +8x
 x = 4 . . . . . . divide by 4