（a+3）的绝对值+4b^2+4b+1=0
（a+3）的绝对值+(2b+1)^2=0
（a+3）的绝对值=0,(2b+1)^2=0
a=-3
b=-1/2
(6b-2a)/(a+b)
=[6*(-1/2)-2*(-3)]/(-3-1/2)
=(-3+6)/(-7/2)
=3/(-7/2)
=-3*2/7
=-6/7