Its not perfect, especially when you see the notation about alienation (i've been learning gismu with Anki and it doesn't have this notation, so I assumed it was mathematical until grabbed that link).
I'm nearly certain this has been remarked on in the past, but there appears to be no gismu for "difference" in the mathematical sense. This is pretty inconsistent, as there're gismu for the other three basic mathematical operators, namely {sumji}, {pilji}, and {dilcu}, for sum, product, and quotient, respectively.
As it stands, there're a few non-gismu solutions, such as {selsumji}. Indeed, cleverly rearranging the places of a sum produces a difference in much the same way as doing so to the places for a product produces a quotient. Why, then, should {dilcu} and not <difference> exist?
If we consider that {vu'u}, the mekso subtraction operator, is based on {vimcu}, we can try to make a subtraction selbri from it. First, we need to remove the agent with zil-, then we need to convert the (thus) x3 with ter-: {terzilvi'u}.
{terzilvi'u} has one obvious problem though: semantically, it isn't purely mathematical. The other three are clearly definitely mathematical, whereas {vimcu} is used for any partial removal of something. "Getting a haircut" is phrased in terms of {vimcu}.
It seems like it'd add consistency if a gismu were simply made up for this rather than try to find other solutions. I (naïvely) suggest {subja}, with -sub- from English & romlang "subtract" and -ja- from Mandarin "jiǎn".
subja = x1 is the difference, x2 minus x3
If there were other suggestions for a potential gismu for this in the past, I'd like to know them. If there were none, I'll probably add this to jbovlaste within a few days.
.i mi'e la tsani mu'o