The distance from a point on the curve to the point (0,2) is given by
sqrt(x^2 + (y-2)^2)
The distance from the line y=4 to a point is given by
They said that the distance from the point on the curve to the line y=4 was double the distance from the point to (0,2), so ...
sqrt(x^2 + (y-2)^2) = 2 * (4-y)
That is how you start the problem. They want you to simplify it as well.