[tex]\it S=\dfrac{1}{2}\cdot|\Delta|\\ \\ \\ \Delta =\ \ \begin{vmatrix}1&2&1\\2&-3&1\\0&-4&1 \end{vmatrix} \stackrel{\ell_2-2\ell_1}{\Longrightarrow}\ \Delta=\ \begin{vmatrix}1&2&1\\0&-7&-1\\0&-4&1 \end{vmatrix}=-7-4=-11\\ \\ \\ S=\dfrac{1}{2}\cdot|-11|=\dfrac{11}{2}=5,5\ u,\ a.[/tex]