2分之x-2【（x-3分之y²）-6分之1（y²-2分之9x）】,
x/2-2[x-y^2/3-(y^2-9x/2)]
=x/2-2[x-y^2/3-y^2+9x/2]
=x/2-2[9x/2-4y^2/3]
=x/2-9x+8y^2/3
=-17x/2+8y^2/3
=-17/2*(-3)+8/3*(9/4)
=51/2+6
=63/2