Work Step by Step
$(3x^2 -7xy+7y^2)-(4x^2 -xy+9y^2)$ $=3x^2 -7xy+7y^2-4x^2 +xy-9y^2$ $=3x^2 -4x^2-7xy +xy+7y^2-9y^2$ $=-x^2-6xy-2y^2$
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