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-84xy
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-84xy
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9x^{2}-42xy+49y^{2}-\left(3x+7y\right)^{2}
Use binomial theorem \left(a-b\right)^{2}=a^{2}-2ab+b^{2} to expand \left(3x-7y\right)^{2}.
9x^{2}-42xy+49y^{2}-\left(9x^{2}+42xy+49y^{2}\right)
Use binomial theorem \left(a+b\right)^{2}=a^{2}+2ab+b^{2} to expand \left(3x+7y\right)^{2}.
9x^{2}-42xy+49y^{2}-9x^{2}-42xy-49y^{2}
To find the opposite of 9x^{2}+42xy+49y^{2}, find the opposite of each term.
-42xy+49y^{2}-42xy-49y^{2}
Combine 9x^{2} and -9x^{2} to get 0.
-84xy+49y^{2}-49y^{2}
Combine -42xy and -42xy to get -84xy.
-84xy
Combine 49y^{2} and -49y^{2} to get 0.
9x^{2}-42xy+49y^{2}-\left(3x+7y\right)^{2}
Use binomial theorem \left(a-b\right)^{2}=a^{2}-2ab+b^{2} to expand \left(3x-7y\right)^{2}.
9x^{2}-42xy+49y^{2}-\left(9x^{2}+42xy+49y^{2}\right)
Use binomial theorem \left(a+b\right)^{2}=a^{2}+2ab+b^{2} to expand \left(3x+7y\right)^{2}.
9x^{2}-42xy+49y^{2}-9x^{2}-42xy-49y^{2}
To find the opposite of 9x^{2}+42xy+49y^{2}, find the opposite of each term.
-42xy+49y^{2}-42xy-49y^{2}
Combine 9x^{2} and -9x^{2} to get 0.
-84xy+49y^{2}-49y^{2}
Combine -42xy and -42xy to get -84xy.
-84xy
Combine 49y^{2} and -49y^{2} to get 0.
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