Which choice shows x + y = 3 in slope-intercept form and the correct graph of the equation?

To be able to graph a y=mx+b, it need to be in y=mx+b. So to answer your question you do the following.
x+y=3
You subtract the x to both side so it looks like this:
x+y=3
-x -x
Final answer is:
y=-x+3