Question 549851
It's best to get both equations in the form y=mx+b in order to graph them.

x+y=3
subtract x from both sides
y=-x+3


x+3y=-3
subtract x from both sides
3y=-x-3
Divide both sides by 3
y=-(1/3)x-1

{{{ graph( 300, 200, -6, 5, -10, 10, x+3, -(1/3)x-1) }}} 

We can see that they intersect at (-3,0)