7a²+4ab-b²-2(2a²-ab+3b²)
=7a²+4ab-b²-4a²+2ab-6b²
=3a²+6ab-7b²
=3x2²+6x2x-1-7(-1)²
=12-12-7
=-7那这题呢？若X=1/2y+2则多项式2（2x-y)的平方+3（y-2x)值为