What is the length of the major axis?
x^2/16+y^2/25=1

The general equation of an ellipse is 

 x^2         y^2
 ---      +      = 1
 a^2         b^2     

where a or b is the semi-major axis depending on values of a^2 and b^2.

So for our ellipse major axis = 2b and b^2 = 25 and b = 5

And major axis = 2b = 2*5 = 10 answer.