The standard form equation of an ellipse with foci on the y-axis is ...
y²/a² +x²/b² = 1
where "a" and "b" are the lengths of the semi-major and semi-minor axes, respectively. If "c" is the distance from the center to the focus, then this relation also holds:
b² +c² = a²
For this ellipse ...
b² + 8² = 17²
289 -64 = b² = 225 . . . . . subtract 8²
b = 15 . . . . . . . . . . . . . . . . . take the square root
