4a²-3a³-2a-6+a³+3a-7+2a³
=(-3+1+2)a³+4a²+(-2+3)a-13=4a²+a-13=-5/2(a=二分之三)
5x²y²+1/4xy-2x²y²-1/6xy-3x²y²=(5-2-3)x²y²+(1/4-1/6)xy=1/12 xy=-1(x=3,y=﹣4)