Suppose you are sitting under an umbrella reading a book. you suddenly look up at the sky to see an aeroplane fly past. explain the sequence of events in your eyes that enable you to see both the printed page in your book and the aeroplane.

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.

: -)

i'd have to say the answer is probable the second option "the geosphere interacting with the atmosphere"