X^2/16+y^2/25=1 the length of the major axis is:

You need to rewrite the equation first and that will tell you the length. 

(x-0)^2 / 4^2 + (y-0)^2 / 5^2

Overtime you try to draw an ellipse you want it to be in this form because that will tell you what your h and k are and the distance from the center.

So we have a center of (0,0) and to plot the other points the 4 ^2 in the denominator tells you that you move 4 units right and 4 unites left.

The 5^2 under the y in the denominator tells you that you move 5 units up and 5 units down. 

So one axis is 8 and the other axis is 10.  10 being the larger one, it is the major axis.