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-9a^{2}-42a-40
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-9a^{2}-42a-40
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9-\left(49+42a+9a^{2}\right)
Use binomial theorem \left(p+q\right)^{2}=p^{2}+2pq+q^{2} to expand \left(7+3a\right)^{2}.
9-49-42a-9a^{2}
To find the opposite of 49+42a+9a^{2}, find the opposite of each term.
-40-42a-9a^{2}
Subtract 49 from 9 to get -40.
9-\left(49+42a+9a^{2}\right)
Use binomial theorem \left(p+q\right)^{2}=p^{2}+2pq+q^{2} to expand \left(7+3a\right)^{2}.
9-49-42a-9a^{2}
To find the opposite of 49+42a+9a^{2}, find the opposite of each term.
-40-42a-9a^{2}
Subtract 49 from 9 to get -40.
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