there are two methods:

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.

Try it.

If you get stuck anywhere in between, I will you the solution.

Keep Smiling

Malay