Either would work, but substitution looks a bit easier.
The graph for these two is shown below.
I just put the equations in just as they are written. They did not have to be changed in any way. DESMOS is a wonderful program. Very powerful.
The blue line is 2x + y = - 6
The red line is y = x + 3
The best way to solve this other than a graph is to use substitution. Put the red graph into the blue one.
2x + y = - 6
2x +(x + 3) = - 6
2x + x + 3 = - 6
3x + 3 = - 6 Subtract 3 from both sides.
3x = - 6 - 3
3x = - 9
x = -9/3
x = -3
Now go back and solve for y
y = x + 3
y = -3 + 3
y = 0
Answer
The solution point is (-3,0) just what the graph shows.