For this one, there are a few steps that will make it easier towards the end.
First lets solve for y
y+3x=8
subtract 3x from both sides
y=-3x+8
Now you are ready to plug in each of the x values and solve for y.
x vaues: -1, 0, 3
Here is what it would look like for each:
y=-3(-1)+8
y=3+8
y=11
y=-3(0)+8
y=8
y=-3(3)+8
y= -9+8
y=-1
So your final answer would be y= {-1, 8, 11}
~be careful putting it into the system, that one is sensitive~
Hope this helps!