②-①5a+b=-1 ④
③-①5a-5b=-5 ⑤
=> 6b=4 ∴b=2/3a=(-1-2/3)/5=-1/3【将b代入④】
=> c=3-2*2/3-4(-1/3)=3
∴a=-1/3；b=2/3； c=3