一个式子a与多项式2x-y的二次方得和恰好等于多项式3x+y的二次方与他的差,则a=

a + (2x - y)^2 = (3x+y)^2 - a
a + 4x^2 - 4xy +y^2 = 3x^2 + 6xy + y^2 - a
2a = -x^2 + 10xy
a = -x^2 / 2 + 5xy