$\text{D(x)}$ = $x^{2}$ - $\text{5x}$ + $\text{4}$
⇔ $x^{2}$ - $\text{4x}$ + $\text{x}$ + $\text{4}$
⇔ $\text{x}$$\text{(x - 4)}$ - $\text{(x - 4)}$ = $\text{0}$
⇔ $\text{(x - 1)}$ $\text{(x - 4)}$ = $\text{0}$
⇔ \(\left[ \begin{array}{l}x-1=0\\x-4=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=1\\x=4\end{array} \right.\)
⇔ $\text{x}$ ∈ {$\text{1}$; $\text{4}$}
