Answer to Where the 2nd Line Went

> implicitplot({x+y=4},x=-1..2,y=-1..2);

[Maple Plot]

Notice that if I eliminate the 1st equation, then we can't see the line. Looking more closely, we see that the problem is that we have limited the
values too much:

> implicitplot({x+y=2,x+y=4},x=-1..2,y=-1..5);

[Maple Plot]