2,6,-2x,3y 的平均数是3
(2+6-2x+3y)/4=3
-2x+3y=4(1)
-5,-3x,12,4y 的平均数是3
(-5-3x+12+4y)/4=3
-3x+4y=5(2)
(1)*3-(2)*2
-6x+9y+6x-8y=12-10
y=2
x=(3y-4)/2=1
xy(x+y)
=1*2*(1+2)
=6