Mk làm mẫu câu a nha
a, => xy+3x-7y-21 = 0
=> (xy+3x)-(7y+21) = 0
=> x.(y+3)-7.(y+3) = 0
=> (y+3).(x-7) = 0
=> y+3=0 hoặc x-7=0
=> x=7 hoặc y=-3
Tk mk nha
\(a)\) \(xy+3x-7y=21\)
\(\Leftrightarrow\)\(x\left(y+3\right)-\left(7y+21\right)=0\)
\(\Leftrightarrow\)\(x\left(y+3\right)-7\left(y+3\right)=0\)
\(\Leftrightarrow\)\(\left(x-7\right)\left(y+3\right)=0\)
\(\Leftrightarrow\)\(\orbr{\begin{cases}x-7=0\\y+3=0\end{cases}}\)
\(\Leftrightarrow\)\(\orbr{\begin{cases}x=7\\y=-3\end{cases}}\)
Vậy \(x=7\) và \(y=-3\)