We know that the equation of a plane parallel to $ x- $ axis is $ by+cz+d=0 $ Since, it passes through the points (2 3 1) and $ (4,-5,3) $ .
$ \therefore $ $ 3b+c+d=0 $ ..(i) and $ -5b+3c+d=0 $ ...(ii)
On solving Eqs. (i) and (ii), we get
$ \frac{b}{1-3}=\frac{c}{-8}=\frac{d}{14} $
$ \Rightarrow $ $ \frac{b}{-2}=\frac{c}{-8}=\frac{d}{14} $
$ \therefore $ Equation of plane is
$ -2y-8z+14=0 $
$ \Rightarrow $ $ y+4z=7 $