（3xy)²(x²-y²）-（4x²y²）²÷8y²+9x²y的四次方
=9x²y²(x²-y²）-2x^4y²+9x²y^4
=9x^4y²-9x²y^4-2x^4y²+9x²y^4
=7x^4y²