设A=4X²-6XY+2Y²+3X-Y,B=2X²-3XY＋Y²-X＋2Y,若X=a,Y=-3,且A-2B=2a,求B的值

设A=4X²-6XY+2Y²+3X-Y,B=2X²-3XY＋Y²-X＋2Y,若X=a,Y=-3,且A-2B=2a,求B的值
设A=4X²-6XY+2Y²+3X-Y,B=2X²-3XY＋Y²-X＋2Y,若X=a,Y=-3,且A-2B=2a,求B的值

设A=4X²-6XY+2Y²+3X-Y,B=2X²-3XY＋Y²-X＋2Y,若X=a,Y=-3,且A-2B=2a,求B的值
解
A-2B
=(4x²-6xy+2y²+3x-y)-2(2x²-3xy+y²-x+2y)
=(4x²-4x²)+(-6xy+6xy)+(2y²-2y²)+(3x+2x)+(-y-4y)
=5x-5y
=5a+15=2a
∴3a=-15
∴a=-5