4(2a^b-ab^2)-5(ab^2+2a^2b)
=8a^2b-4ab^2-5ab^2-10a^2b
=-2a^2b-9ab^2
=-ab(2a-9b)
=- -1/2x1/3(-1/2x2-1/3x9)
=1/6 (-1-3)
=-2/3