How is the dark dorsal side and the light ventral side of a frog adaptive?

The advantages of different skin colours of the frog is in survival, they help them camouflage from predators. If a predator is below them, the brighter ventral site simulates the bright sky (or water surface). If the predator is above, then dorsal side helps the frog to blend into the surrounding ground environment (or floating vegetation). 

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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