Say that you start from a number
. Then, you subtract
and
from
, which means that the expression is

If we want this expression to be negative, we have

So, in order for the expression to be negative, the two numbers we are subtracting, combined, must be more than the starting number.
For example, if we start from 10 and we subtract 4 and then 8, we have

which is negative, because 4 and 8 combined are more than 10