Taxonomy is:

a. a very recent science
b. not a science
c. a very old science
d. an exact science

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.

: -)

"d.) carbon dioxide and hydrogen sulfide"