$\begin{array}{l}3c) \begin{cases}1,7x - 2y = 3,8\\2,1x +5y = 0,4\end{cases}\\\Leftrightarrow \begin{cases}y= \dfrac{1,7x - 3,8}{2}=0,85x - 1,9\\2,1x +5(0,85x - 1,9) = 0,4\end{cases}\\\Leftrightarrow \begin{cases}y=0,85x - 1,9\\6,35x - 9,5 = 0,4\end{cases}\\\Leftrightarrow \begin{cases}y=0,85.\dfrac{198}{127} - 1,9=-\dfrac{73}{127}\\x= \dfrac{9,5+0,4}{6,35} = \dfrac{198}{127}\end{cases}\\3d)\begin{cases}-x+2y = 6-2\sqrt{5}\\(5+\sqrt{2})x+y = 3-\sqrt{5}\end{cases}\\\Leftrightarrow \begin{cases}y=\dfrac{6-2\sqrt{5}+x}{2} = 3 - \sqrt{5} + \dfrac{1}{2}x\\(5+\sqrt{2})x+(3 - \sqrt{5} + \dfrac{1}{2}x) = 3-\sqrt{5}\end{cases}\\\Leftrightarrow \begin{cases}y = 3 - \sqrt{5} + \dfrac{1}{2}x\\(5 +\sqrt{2}+\dfrac{1}{2})x = 0\end{cases}\\\Leftrightarrow \begin{cases}x = 0\\y = 3 - \sqrt{5} \end{cases}\end{array}$
