An ellipsoid is a 3D shape whose cross sections are ellipses. A lithotripter is a device in the shape of a half-ellipsoid that is used to crush kidney stones with shockwaves. An emitter is placed at the focus closer to the lithotripter and the kidney stone is placed at the other focus. If the equation 7921 + 1648.36 = 1 models the elliptical cross section of the lithotripter (in cm) shown below, how far from the emitter should the stone be placed