[tex]\it 5-x+4=19 \Rightarrow 9-x=19 \Rightarrow -x=19-9 \Rightarrow -x=10|_{\cdot(-1)} \Rightarrow \\ \\ \Rightarrow x=-10\not\in\mathbb{N} \Rightarrow A=\varnothing \Rightarrow cardA=0 \Rightarrow A=finit\breve a\\ \\ \rule{198}{0.3}\\ \\ x-7<3|_{+7}\Rightarrow x<10 \Rightarrow B=\{0,\ 1,\ 2,\ 3,\ ...,\ 9\} \ \ finit\breve a\\ \\\rule{198}{0.3}\\ \\ x=3n \Rightarrow C=\{0,\ 3,\ 5,\ 9,\ 12,\ 15,\ ...\ \}\ \ infinit\breve a\\ \\ \rule{198}{0.3}\\ \\ La\ D,\ n =?.[/tex]
