xy+3x-7y = 21
x.(y+3)- 7y-21 = 0
x.(y+3)- 7.(y+3) =0
(x-7).(y+3) =0
=> x-7 = 0 hoặc y+3 = o
TH 1: x-7 = 0 => x = 7
TH2: x+3 = 0 => x= -3
Ta co:x.y+3.x-7.y=21
=>x.(y+3)-7.y=21
=>x.(y+3)-7.y-21=0
=>x.(y+3)-(7.y+21)=0
=>x.(y+3)-7.(y+3)=0
=>=>(x-7).(y+3)=0
=>x-7=0 hoac y+3=0
=>x=7 hoac y=(-3)
Vay voi x=7 thi y thoa man moi gia tri(y thuoc Z);y=(-3) thi x thoa man moi gia tri(x thuoc Z