Human activities can add which of the following to the atmosphere a. heat b. carbon dioxide c. oxygen d. water vapor

B, when humans exhale we put out carbon dioxide.

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.

: -)