Subduction
What happens to the material that subducts? Where does it go?

Where two tectonic plates meet at a subduction zone, one bends and slides underneath the other, curving down into the mantle. (The mantle is the hotter layer under the crust.) ... At a subduction zone, the oceanic crust usually sinks into the mantle beneath lighter continental crust.

Explanation:

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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