Question

How do you subtract `-3.1 - 21.9`?

Answer 1

Mostly likely: `-3.1=-21.9=color(blue)(-25.0)`
...but see below

Explanation:

Normal evaluation is from left to right;
so `-3.1-21.9`
would commonly be evaluated as
`color(white)("XXX")(-3.1)-(21.9)`
`color(white)("XXX")=color(white)("xx")-3.1color(white)("xxx")3.1` units to the left of zero on the number line
`color(white)("XXX"=)-ul(color(white)("x")21.9color(white)("xxx")`move `21.9` units further to the left
`color(white)("XXX")=color(white)("x")-25.0color(white)("xx")` now a total of `25` units to the left of zero.

`ul("However")`
Sometimes evaluation is done right to left:
So `-3.1-21.9`
would be evaluated as `-(3.1-21.9)=-(-17.2)=+17.2`