Let y = 0 to solve for the x-intercepts of the circle.
.: A is (±4, 0)
Now let B(h, k) be a point on the circle, so that AB forms a chord. Let M be the midpoint of AB.
.: M is [(h ± 4)/2, k/2]
i.e. The locus of M is defined by this set of parametric equations:
x = (h ± 4)/2 ...(1)...