for an ellipse, that is for
with a center at (h , k), first off, let's recall that for ellipses both fractions are positive.
The major axis occurs with the fraction with the larger denominator, well hmm the larger denominator is 16, and since we know it has a horizontal orientation or is namely of a horizontal major axis, that means that "16" will be under the fraction with the "x" with it, Check the picture below.