just plot the points on the graph so if x+y=-6 just make a list of all of the points that will work (remember that the left-to-right line is x and the up-to-down line is y, and right and up is positive and left and down is negative)
so make a list (in graphs we write [x,y] or (x,3)) so here are some points that will work
(-3,-3)
(-4,-2)
(-2,-4)
(-1,-5)
(-5,-1)
(0,-6)
(-6,0)
(-7,1)
(1,-7)
and so on, just plot those points and more that will work for x+y=-6 and you should be good,
there is another way that this is supposed to be done but this way is easier