In the countercurrent exchange system of fish gills, blood and water flow in opposite directions. blood flow in the gills reverses direction with every heartbeat. blood and water are separated by a thick polysaccharide barrier. blood and water flow in the same direction.

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 think its the first one

the titanic would be the correct answer

a chain link fence because there so tiny even my albino donkey can't get out