∵x=根号3+2,y=根号3-2
∴x-y=4,x-2=根号3
2x*2-2xy+y*2-4x+a
=x²-2xy+y²+x²-4x+a
=(x-y)²+(x-2)²+a-4
=4²+3+a-4
=15+a