Answer: Lets rewrite our equation.
x + y + kz - 5z =k
x + y -5z = k(1-z)
k = [tex]\frac{x + y - 5z}{1 - z}[/tex]
so the only problem with the equation is that z can't be equal to one.
but x and y are free, so you have two free variables and one equation,
that means that for any k the system has infinitely many solutions.