$a)\underline{+\{\begin{array}{l}3x+y=5\\ 2x-y=0\end{array}}$    

                        $5x=5\mid :5$   

                           $x=1$   

$\{\begin{array}{l}x=1\\ 3\cdot 1+y=5\end{array}$   

$\{\begin{array}{l}x=1\\ y=2\end{array}$  

$b)\underline{+\{\begin{array}{l}2x-y=5\\ x+y=4\end{array}}$   

                      $3x=9\mid :3$  

                         $x=3$  

$\{\begin{array}{l}x=3\\ 3+y=4\end{array}$  

$\{\begin{array}{l}x=3\\ y=1\end{array}$  

$c)\underline{\{\begin{array}{l}-x-y=-6\\ -x+y=2\end{array}}$     

                    $-2x=-4\mid :\left(-2\right)$  

                           $x=2$  

$\{\begin{array}{l}x=2\\ -2+y=2\end{array}$  

$\{\begin{array}{l}x=2\\ y=4\end{array}$  

$d)\{\begin{array}{l}4x-5y=-4\\ -2x+3y=1.1\mid \cdot 2\end{array}$  

$\underline{+\{\begin{array}{l}4x-5y=-4\\ -4x+6y=2.2\end{array}}$   

                           $y=-1.8$  

$\{\begin{array}{l}y=-1.8\\ -2x+3\cdot \left(-1.8\right)=1.1\end{array}$  

$\{\begin{array}{l}x=-3.25\\ y=-1.8\end{array}$  

$e)\{\begin{array}{l}-2x+y=0\mid \cdot 3\\ 3x-3y=1\end{array}$  

  $\underline{+\{\begin{array}{l}-6x+3y=0\\ 3x-3y=1\end{array}}$  

                   $-3x=1\mid :\left(-3\right)$  

                          $x=-\frac{1}{3}$  

$\{\begin{array}{l}x=-\frac{1}{3}\\ 3\cdot \left(-\frac{1}{3}\right)-3y=1\end{array}$

$\{\begin{array}{l}x=-\frac{1}{3}\\ -3y=2\end{array}$  

$\{\begin{array}{l}x=-\frac{1}{3}\\ y=-\frac{2}{3}\end{array}$  

$f)\{\begin{array}{l}3x-2y=-12\mid \cdot \left(-2\right)\\ 2x-5y=-19\mid \cdot 3\end{array}$  

$\underline{+\{\begin{array}{l}-6x+4y=24\\ 6x-15y=-57\end{array}}$   

                $-11y=-33\mid :\left(-11\right)$  

                         $y=3$  

$\{\begin{array}{l}y=3\\ 3x-2\cdot 3=-12\end{array}$  

$\{\begin{array}{l}3x=-6\\ y=3\end{array}$

$\{\begin{array}{l}x=-2\\ y=3\end{array}$