Question

$$\begin{gathered} \frac{{\left(1-x^{}\right)}^3{\left(1-x^{}\right)}^3\left(1-x^4\right)}{{\left(1-x\right)}^5} \\ =\frac{{\left(1-x^{}\right)}^3{\left(1-x^{}\right)}^3\left(1-x^{}\right)\left(1+x^2+x\right)\left(1-x\right)\left(1+x\right)\left(1+x^2\right)}{{\left(1-x\right)}^5} \end{gathered}$$
= (1 + x)4(x⁴ + x³ + 2x² + x + 1)
= ⁴C₁ + ⁴C₂ + 2⁴C₃ + ⁴C₄ = 19