we have
equation 1 [tex]2x+y=0[/tex]
equation 2 [tex]y=3[/tex]
substitute the equation 2 in equation 1
[tex]2x+[3]=0 \\ 2x=-3 \\ x=- \frac{3}{2} [/tex]
the solution is the point
[tex](- \frac{3}{2} ,3)[/tex]
using a graph tool
see the attached figure
