[tex]\displaystyle\bf\\-\frac{3}{7}a b^{3} \cdot\Big(-2 \frac{1}{3} a^{3} b^{2} \Big)=\underbrace{-\frac{3}{7} \cdot\Big(-\frac{7}{3} \Big)}_{1}\cdot a^{1+3}\cdot b^{3+2} =a^{4} b^{5}[/tex]