7. The straight line \(l\) passes through the point \(P ( - 3,6 )\) and the point \(Q ( 1 , - 4 )\).
- Find an equation for \(l\) in the form \(a x + b y + c = 0\), where \(a , b\) and \(c\) are integers.
The straight line \(m\) has the equation \(2 x + k y + 7 = 0\), where \(k\) is a constant.
Given that \(l\) and \(m\) are perpendicular, - find the value of \(k\).