a: \(\Leftrightarrow5\sqrt{x+3}-2\sqrt{x+3}+\sqrt{x+3}=8\)
\(\Leftrightarrow4\sqrt{x+3}=8\)
=>x+3=4
hay x=1
b: \(\Leftrightarrow\left\{{}\begin{matrix}3x-5y=17\\3x+21y=-9\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}-26y=26\\x+7y=-3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=-1\\x=-3-7y=-3-7\cdot\left(-1\right)=7-3=4\end{matrix}\right.\)
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