Perpendicular lines

$\mbox{If a line has slope }m_1 \mbox{ then a line perpendicular to that line will have slope } m_2 = -\dfrac 1 {m_1} \\ 7y = 9x + 1 \\ y = \dfrac 9 7 x + \dfrac 1 7\\ \mbox{This line has slope }m_1 = \dfrac 9 7 \\ -3y = x - 24 \\ y = -\dfrac 1 3 x + 8 \\ \mbox{This line has slope }m_2 = -\dfrac 1 3 \\ -\dfrac 7 9 \neq - \dfrac 1 3 \\ \mbox{So the two lines are not perpendicular.}$