2. if the optimal ph range of most enzymes is between 6 and 8, is there possibly some
advantage to having certain enzymes, such as pepsin, active in the low ph of the
stomach?

i'd say that the moon rock is about 2.2 billion years old.

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.

: -)