\[\left( - \frac{3}{4}\text{abc} \right)^{3} = \left( - \frac{3}{4} \right)^{3}a^{3}b^{3}c^{3} =\]
\[= - \left( \frac{3}{4} \right)^{3}a^{3}b^{3}c^{3} = - \frac{27}{64}a^{3}b^{3}c^{3}.\]