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

4-y

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.