It is just the plane containing the midpoint with normal
Find an equation for the plane, each of whose points is equidistant from (-1, -4, -2) and (0, -2, 2).
I tried to work on this by setting two distance formulas equal to each other, but I didn't know how to handle the absolute value on both sides, and the d term kept canceling out.
Also, I'm really hoping the question is referring to points and not vectors using that notation.