x + 280 + 720 + 200 = 200 +800 -0
x + 1200 = 1000
x = 1000 - 1200
x = -200
vậy x= -200
x+280+720+200=200+800-0
x+280+720+200=1000
x+280+720=1000-200
x+280+720=800
x+280=800-720
x-280=80
x=80-280
x=-200
x+280+720+200=200+800-0
x+1200=1000
x=1000-1200
x=-200