@box said:

Actually Cotty a mesh of the type he is talking about by it's very nature cannot be a solid.

However, you can intersect a plane with the mesh and it will 'Cut' the the edges along that line, you can then manually delete the parts you don't want.

is there a video tutorial on that ?

Is there a way where i can slice or subtract the rectangle from the mesh round object?
the rectangle is solid but the round object is the mesh .
is there anyway or any plugin ? i can't use the subtract since it is a mesh round object