[4a^(-2)b^(-4)/16(a^(3)b)^(-1)]÷[8a^(-3)b/32(ab^(3))^(-1)]
=[(1/4)*a^(-2+3)*b^(-4+1)]÷[(1/4)*a(-3+1)*b^(1+3)]
=[(1/4)*a*b^(-3)]÷[(1/4)*a(-2)*b^4]
=a(1+2)*b(-3-4)
=a^3b^(-7)
[(18x-3xy)/6x^2]÷[(4xy+8x^2)/2xy]
=[(18x-3xy)/6x^2]×[2xy/(4xy+8x^2)]
=2xy(18x-3xy)/[6x^2(4xy+8x^2)]
=y(18x-3xy)/[3x(4xy+8x^2)]
=3xy(6-y)/[6x^2(2y+4x)]
=y(6-y)/[2x(2y+4x)]
=(6y-y^2)/(4xy+8x^2)