It is generally thought that four critical innovations arose during the evolution of our ape-like ancestors into humans. all of the following are innovations except

answer: 2

explanation:

yes

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)