Find the height of the dome at the center

The dome of a whispering gallery has the form of a semi elliptical so that two persons standing at the foci will be able to hear each other. This is because sound waves from one focus, when they reach the semi - elliptical ceiling, bounce off to the other focus. One such whispering gallery has a focus 3 m from an end of the semi - ellipse, and the other focus is 17 m away from the same end. What is the height of the dome at the center? O 21 O V26 41 W51