These two lines are coincident. Their equations are equivalent.[br][br][center][math]y=10-x\Longleftrightarrow y+x=10[/math][/center][center][/center][br][br]All ordered pairs in one relation are also in the other. Therefore there are infinitely many solutions to the system[br][center][br][math]\begin{cases}y+x=10\\y=10-x\end{cases}[/math][/center][br][br]The infinitely many solutions are all of the ordered pairs that lie on the line.