Say the number line has 9 coordinates, coordinates are just lines on a number line, now start at the middle coordinate which is the 5th coordinate now there are 4 coordinates in front of it and 4 coordinates behind it so since you are starting at the 5th coordinate just count backwards (the coordinates) 3 times that is the third coordinate and that is where you Graph point P at -3 on a number line!
Hope it helps! :D :D