The vertebrates in which blood flows directly from respiratory organs to body tissues without first returning to the heart are the a. fishes b. birds c. mammals d. reptiles e. amphibians

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.

: -)

but there is no

i guess you forgot putting it.