5a^xy^2和-2/3a^3y^b,可合并
说明他们的同类项
所以a^x=a^3 => x=3
y^2=y^b => b=2
-1/3x^3+2b^2-x-1/2b-4
=-1/3*3^3+2*2^2-1/2*2-4
=9+8-1-4
=12