原式=4x²+6x³÷3x²-12x^4÷3x²=4x²+2x-4x²=2x
(-2x)^2+(6x^3-12x^4)/(3x^2)=4x^2+(6x^3)/(3x^2)-(12x^4)/(3x^2)=4x^2+2x-4x^2=2x