check the picture below, so the ellipse looks more or less like so.
since the major axis this time is over the x-axis, yeap, you guessed it, "a" will be under the "x" fraction.
notice the graph, we know b = 3, and c = 4.
[tex]\bf \textit{ellipse, horizontal major axis}
\\\\
\cfrac{(x- h)^2}{ a^2}+\cfrac{(y- k)^2}{ b^2}=1
\qquad
\begin{cases}
center\ ( h, k)\\
vertices\ ( h\pm a, k)\\
c=\textit{distance from}\\
\qquad \textit{center to foci}\\
\qquad \sqrt{ a ^2- b ^2}
\end{cases}\\\\
-------------------------------[/tex]
[tex]\bf \begin{cases}
h=0\\
k=0\\
b=3\\
c=4
\end{cases}\implies \cfrac{(x- 0)^2}{ a^2}+\cfrac{(y- 0)^2}{ 3^2}=1
\\\\\\
c=\sqrt{ a ^2- b ^2}\implies c^2=a^2-b^2\implies 4^2=a^2-3^2
\\\\\\
4^2+3^2=a^2\implies \sqrt{4^2+3^2}=a\implies \boxed{5=a}
\\\\\\
\cfrac{(x- 0)^2}{ 5^2}+\cfrac{(y- 0)^2}{ 3^2}=1\implies \cfrac{x^2}{25}+\cfrac{y^2}{9}=1[/tex]
