How can you face a problem if your problem is your face?

First you need to look into the mirror and really ponder your existence. Then maybe plastic surgery.

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.

: -)

the correct answer is:

a population changes in response to a change in the environment.