Answer
$$ y=5.$$
Work Step by Step
Since the plane is parallel to the $ xz $-plane
$$ y=0$$
then its normal vector $ n $ is $\langle 0,1,0\rangle $
and, using the fact that the plane passes through $(3,5,-9)$, the equation of the plane is given by
$$0(x-3)+(y-5)+0(z+9)=0\Longrightarrow x-5=0$$
Hence the equation is given by
$$ y=5.$$
