do you mean:
For my first 'adventure', I'm trying to make a very simple piece. 20mm x 12mm x 205mm, with a 7mm hole at one end centered at 12.5mm from the end and a 7mm hole/slot at the other end centered at 11.5mm from the end (12.5mm would work too). Pretty simple to make, you'd think. Not so much.
20 long, 12 wide and 205 tall ?

in which case - try this:
$fn=100;

difference() {
cube([20,12,205]);

translate([10,12,193.5]) rotate([90,0,0]) cylinder(d=7, h=13);
translate([10,12,12.5]) rotate([90,0,0]) cylinder(d=7, h=13);

}// end diff

Now a hole is not a slot - so which do you mean, I put a hole :-)
openscadwidget.jpg


For the record dimensions are usually given: x:y:z Length:width:height.
I've seen quite a few americans who put z first, or sometimes in the middle - or sometimes even use inches :-)