Afeature that freed reptiles from dependence on water for reproduction is

b. the carbon cycle ensures living things have a source of hydrogen

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.

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