Caculate the number of trees

Hello everybody
If I have a land and I need to know the numbers of trees needed to be planted. How can i achieve that?
The land is ractangular . The legnth of the land is 1000 m and the width is 500 m
The distance between each tree should be 6 m

If UDF function the solution would be perfect
UDFName(Length,Width,Distance)
Thanks advanced for help

=1000*500/6^2

Thanks Mr. shg for this quick reply
Is it a rule ? Can you simply exaplain the logical steps of doing that?

It's the total area divided by the area occupied per tree.

Thanks for simple and useful explain