直接将-1/3带入到方程中
2*a^2-a*(27-5*a)-7*(a+1)*(a+2)
=2a^2-27a+5a^2-7(a^2+3a+2)
=2a^2-27a+5a^2-7a^2-21a-14
=-48*a-14
=-48*(-1/3)-14
=16-14
=2
>> a=-1/3
2*a^2-a*(27-5*a)-7*(a+1)*(a+2)
ans =
2.0000