I tried it two different ways and am not sure if either is correct:
2a=10 so a=5 (sum of focal radii)
Center is (0,0) halfway between foci
b^2=a^2-c^2 or b^2 = 16
giving me the equation x^2/16 + y^2/25 = 1
Then I tried it like the textbook by using the distance formula:
squareroot of [x² + (y+3)²] + squareroot of [x² + (y-3)²]=10
When I tried to square both sides, I ended up getting it simplified to x²+y²= 41
Obviously totally different answers—not sure which is correct and why the other is incorrect.
Thank you for your assistance.