These may help you. You can square both sides.
Hello, Thomas!
Another approach . . .
is the distance from to the point (4,0).If satisfies
show that: .
is the distance from to the point (-4,0).
If the sum of the distances to two fixed points is a constant,
. . the locus of is an ellipse, whose equation can be derived
. . with the Distance Formula (and a lot of algebra).