Answer
$\displaystyle \frac{x^{2}}{36}+\frac{y^{2}}{9}=1$
Work Step by Step
Major axis horizontal: $\displaystyle \frac{(x-h)^{2}}{a^{2}}+ \displaystyle \frac{(y-k)^{2}}{b^{2}}=1$
Major and minor axes have lengths $2a$ and $2b$
$2a=12 \Rightarrow a=6$
$2b=6\Rightarrow b=3$
Center: $(h,k)=(0,0)$
$\displaystyle \frac{x^{2}}{6^{2}}+\frac{y^{2}}{3^{2}}=1$
$\displaystyle \frac{x^{2}}{36}+\frac{y^{2}}{9}=1$
