(1)
x得平方与绝对值2x-y的平方+2互为相反数：x^2+|2x-y^2+2|=0
∴x^2 = |2x-y^2+2| = 0
∴x=0,y^2=2
a=2x^2+xy+3y^2 = 0+0+3*2=6
b=x^2-xy+2y^2 = 0-0+2*2=4
2a-3(2b-a) = 2a-6b+3a = 5a-6b = 5*6-6*4 = 6
(2)
若x^2+y^2=4,xy=-2
a-b=【2x^2+xy+3y^2】-【x^2-xy+2y^2 】
= x^2+y^2+2xy
= 4+2*(-2)
= 0