a) $\{\begin{array}{l}x+y=2\\ x-y=0\end{array}$   Mamy x = 1 i y =1,bo

$\{\begin{array}{l}1+1=2\\ 1-1=0\end{array}$  

b) $\{\begin{array}{l}2x+y=15\\ x=y\end{array}$   Mamy x = 5 i y=5 , bo

$\{\begin{array}{l}2\cdot 5+5=15\\ 5=5\end{array}$

c) $\{\begin{array}{l}y=x+5\\ 581x+y=5\end{array}$   Mamy x = 0 i y=5, bo

$\{\begin{array}{l}5=0+5\\ 581\cdot 0+5=5\end{array}$