in this problem we have two planes.....
2x-3y+6z-5=0 ...............(1)
6x-9y+18z+20=0
(or) 2x-3y+6z+(20/3) = 0
so we have

and

substituting the values in the formula we get
distance =

distance = 5\3
this formula is given in our text book .. so we can use it in board exam.........