6. The function f is defined by
$$f ( x ) = \frac { 5 x - 3 } { x - 4 } \quad x > 4$$
- Show, by using calculus, that f is a decreasing function.
- Find \(\mathrm { f } ^ { - 1 }\)
- Show that \(\mathrm { ff } ( x ) = \frac { a x + b } { x + c }\) where \(a , b\) and \(c\) are constants to be found.
- Deduce the range of ff.