[tex]\dfrac{\left(\dfrac{4}{3}ab^3\right)\left(\dfrac{6}{2}a^4\right)}{a^5}=\dfrac{\left(\dfrac{\not4^2}{\not3_1}\cdot\dfrac{\not6^2}{\not2_1}\right)(aa^4)(b^3)}{a^5}=\dfrac{4a^5b^3}{a^5}=\boxed{4b^3}[/tex]
