Intermediate Algebra (12th Edition)

We know that two parallel lines have the same slope ($m_1=m_2$). We also know that perpendicular lines have negative reciprocal slopes ($m_1=-\frac{1}{m_2}$). Thus a line parallel to a line with slope $m_1=-4/9$ has slope $m_2=-4/9$. And a line perpendicular to the line with slope $m_1=-4/9$ has a slope $m_2=-\frac{1}{-4/9}=9/4$.