\(\left(x+3\right)^2-x\left(x-4\right)=21\)
\(\Rightarrow x^2+6x+9-x^2-4x=21\)
\(\Rightarrow2x-9=21\)
\(\Rightarrow2x=30\)
\(\Rightarrow x=15\)
Vậy \(x=15\)
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\(\left(x+3\right)^2-x\left(x+4\right)=21\)
\(x^2+6x+9-\left(x^2+4x\right)=21\)
\(x^2+6x+9-x^2-4x=21\)
\(\left(x^2-x^2\right)+\left(6x-4x\right)+9=21\)
\(2x+9=21\)
\(2x\) \(=21-9=12\)
\(x\) \(=12:2=6\)
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