[tex]\text{Let}\ k:y=m_1x+b_1\ \text{and}\ l:y=m_2x+b_2,\ \text{then}\\\\l\ \parallel\ k\iff m_1=m_2\\\\\text{Convert the equation to the slope-intercept form}\ y=mx+b.\\\\2x+y=4\qquad\text{subtract 2x from both sides}\\\\y=-2x+4\to m_1=-2\\\\Amswer:\ \boxed{slope=-2}[/tex]
