Answer
$$ x=4.$$
Work Step by Step
Since the plane is parallel to the plane
$$ x=3$$
then its normal vector $ n $ is $\langle 1,0,0\rangle $
and, using the fact that the plane passes through $(4,1,9)$, the equation of the plane is given by
$$(x-4)+0(y-1)+0(z-9)=0\Longrightarrow x-4=0$$
Hence the equation is given by
$$ x=4.$$
