∵ x+y=4 ,xy=3
∴ x+y=xy+1
∴ x=xy-y+1 ,y=xy-x+1
∴ 3x2+3y2
=3(xy-y+1)2+3(xy-x+1)2
=6xy-6y+6+6xy-6x+6
=12xy-6(x+y)+12
=12*3-6*4+12
=36-24+12
=24