X-a的绝对值+（y+4）的平方=0
绝对值和平方都是非负值,只有他们同时为0时,和才为0
所以x-a=0,y+4=0
=> x=a,y=-4
A-2B=2x^2-4xy+6y^2+3x-y-2x^2+4xy-6y^2+4x-2y
=7x-3y=3a
=>7x+12=3a
=>7a+12=3a
=>4a=-12
=>a=-3