[tex]\bf 18. \ \ \it \begin{vmatrix} \it 3x&\it4\\ x&\it2\end{vmatrix}=4 \Rightarrow 6x-4x=4 \Rightarrow 2x=4 \Rightarrow x=2[/tex]
[tex]\bf 19. \ \ \it \begin{vmatrix} \it x&\it1&\it1\\ \it1&\it x&\it1\\\it1&\it1&\it2\end{vmatrix}=12 \Rightarrow x(2x-1)-2+1+1-x=12 \Rightarrow2x^2-x-x=12 \Rightarrow\\ \\ \\ \Rightarrow2x^2-2x=12|_{:2} \Rightarrow x^2-x=6 \Rightarrow x(x-1)=6 \Rightarrow x\in\{-2,\ \ 3\}[/tex]
[tex]\bf C2.\ \ \it D=(1-\sqrt2)(1+\sqrt2)+(\sqrt3-1)(\sqrt3+1)=1-2+3-1=1[/tex]
[tex]\bf C3.\ \ \it det(AB)=\begin{vmatrix}\it-4&\it-3\\\ \it0&\ \it0\end{vmatrix}=0[/tex]
