Consider the three planes

     $P_1:3x+15y+21z=9$

      $P_2:x-3y-z=5$            

       $P_3:2x+10y+14z=5$

            Then , which one  of  the following is true ?

The direction ratios of normal to the plane$ax+by+cz+d=0$ are $〈a,b,c〉$

The direction ratios of normal to the plane$P_1:3x+15y+21z=9$ are $〈1,5,7〉$

The direction ratios of normal to the plane$P_2:x-3y-z=5$ are $〈1,-3,-1〉$

The direction ratios of normal to the plane$P_3:2x+10y+14z=5$ are $〈1,5,7〉$

Observing the above three planes, direction ratios of normal lines of the planes $P_{1 } and P_3$ are proportional. 

Hence, the planes $P_1,P_3$ are parallel.