Mark M. answered 11/16/19
Retired math prof. Very extensive Precalculus tutoring experience.
Since the center is (0,0) and a focus lies on the x-axis, the equation has the form:
x2 / a2 + y2 / b2 = 1, where c2 = a2 - b2
c = distance from the center to either focus = 6
(0,8) is on the ellipse and is directly above the center. So, (0,8) is an endpoint of the minor axis.
Distance from the center to (0,8) = 8 = (1/2)(length of minor axis) = b/2. So, b = 16.
Since c2 = a2 - b2 and b = 16 and c = 6, a2 = 36 + 256 = 292
Equation of ellipse: x2 / 292 + y2 / 256 = 1