5. $$ \frac{7m}{9} \cdot \frac{a^2 - ab}{21} $$

$$\frac{7m}{9} \cdot \frac{a^2 - ab}{21} = \frac{7m}{9} \cdot \frac{a(a-b)}{21} = \frac{7ma(a-b)}{9 \cdot 21} = \frac{7ma(a-b)}{189} = \frac{ma(a-b)}{27}$$