\[1\frac{3}{4}a^{2}b\left( 4b^{2} - \frac{8}{7}ab + \frac{16}{21}a^{3} \right) =\]
\[= \frac{7}{4}a²b \cdot \left( 4b^{2} - \frac{8}{7}ab + \frac{16}{21}a^{3} \right) =\]
\[= 7a^{2}b^{3} - 2a^{3}b^{2} + \frac{4}{3}a^{5}b =\]
\[= 7a^{2}b^{3} - 2a^{3}b^{2} + 1\frac{1}{3}a^{5}b\]