24. \(\left(\frac{3}{4}b-\frac{2}{5}\right)^2\)

\(\left(\frac{3}{4}b-\frac{2}{5}\right)^2=\left(\frac{3b}{4}\right)^2-2\cdot\frac{3b}{4}\cdot\frac{2}{5}+\left(\frac{2}{5}\right)^2=\frac{9b^2}{16}-\frac{3b}{5}+\frac{4}{25}\).

Ответ: \(\frac{9b^2}{16}-\frac{3b}{5}+\frac{4}{25}\).