Question 160641
y=x+4 (red line)
y+x=4
y=-x+4 (green line)
{{{ graph( 300, 200, -6, 5, -10, 10, x +4, -x +4) }}} (graph 300x200 pixels, x from -6 to 5, y from -10 to 10, of TWO functions x +4 and -x +4). 
SOLUTION=(0,4).