help with gm mogrify

hello,
I've got some images that are not quite big enough.
Want the image to stay as is 2400x1800 and add more white to the bottom 
so it's 2400x1900
I've done it before but forgotten what I did.
Is some combination of "-resize -extent -gravity -size" but I'm 
uncertain which applies to the original image and which the new pixels.
I read that -size is just a hint to speed things up a bit on
http://www.graphicsmagick.org/mogrify.html earlier but that seems 
unavailable just now.
I can experiment until achieve result but was hoping someone could give 
me the line and a brief explanation what applies to what.

graphicsmagick is really quick.

cheers

mick

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