How is this C?

Given:

x^2+y^2 = 81 \tag{1}

x^2-y^2 =0\tag{2}

From eq 2

x^2=y^2\tag{3}

Now, if both x and y are the same then subtracting them would result in zero

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Alternatively, since we know that a^2-b^2 = (a+b)*(a-b), the expression x^4-y^4 can also be simplified into x^4-y^4 = (x^2 + y^2)*(x^2 - y^2) = 81*0 = 0.

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ahhh, I didn’t see that! This makes complete sense, thank you.