either use the direct formula
or use your basic concepts.
I will tell you the second method.
Imagine a point P on the plane.
Join O and P.
Find the drs of OP.
For OP to be the shortest distance botween two planes, OP should have the drs that the normal to the plane has.
Using this, use can find your answer.
If you get stuck anywhere in between, I will you the solution.