7. The point \(A\) has coordinates \(( - 1,5 )\) and the point \(B\) has coordinates \(( 4,1 )\).
The line \(l\) passes through the points \(A\) and \(B\).
- Find the gradient of \(l\).
- Find an equation for \(l\), giving your answer in the form \(a x + b y + c = 0\) where \(a , b\) and \(c\) are integers.
The point \(M\) is the midpoint of \(A B\).
The point \(C\) has coordinates \(( 5 , k )\) where \(k\) is a constant.
Given that the distance from \(M\) to \(C\) is \(\sqrt { 13 }\) - find the exact possible values of the constant \(k\).