Answer
Fill the blanks with $5, 3, x.$
Work Step by Step
Observe that the denominator under $x^{2}$ is greater than under $y^{2}.$
$a^2=25,\ \ b^2=9,$
$a=5, \ \ \ \ b=3.$
The major axis is along the $x$-Axis.
By the corresponding theorem,
An equation of the ellipse with center at $(0,0),$
foci at $(-c,0)$ and $(c,0),$ and vertices at $(-a,0)$ and $(a,0)$ is
$\displaystyle \frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1\quad $ where $a \gt b \gt 0$ and $b^{2}=a^{2}-c^{2}$
Fill the blanks with $5, 3, x.$