Find the equation of the ellipse whose foci are (3,0) and (-3,0) and whose sum of focal radii is 10 ???
First look at Mathwords: Focal Radius
So I will assume your starting point is $\displaystyle \sqrt{(x + 3)^2 + y^2} + \sqrt{(x - 3)^2 + y^2} = 10$.