What is the form of all points (x, y, z) in the y,z-plane?
What is the form of all points (x, y, z) on the x-axis?
What is the three-dimensional Distance Formula?
Plug those two points into the Distance Formula, set the one distance equal to twice the other distance, and simplify to get the equation. (Squaring both sides will likely be your first step.)
Eliz.