Given:
[tex]\begin{gathered} x+y=3 \\ y=-x+3 \end{gathered}[/tex]given solution is (0,3)
[tex]\begin{gathered} (0,3) \\ so\text{ the value x=0} \\ y=3 \end{gathered}[/tex]check for sol. put the value in equation then:
[tex]\begin{gathered} x+y=3 \\ x=0,y=3 \\ 0+3=3 \\ 3=3 \end{gathered}[/tex]for another equation :
[tex]\begin{gathered} y=-x+3 \\ x=0,y=3 \\ 3=0+3 \\ 3=3 \end{gathered}[/tex]so the solution of x and y is right.