Question 9 - A-Level Maths

9 The equation of a curve is \(y = 4 + 5 x + 6 x ^ { 2 } - 3 x ^ { 3 }\).

Find the set of values of \(x\) for which \(y\) decreases as \(x\) increases.
It is given that \(y = 9 x + k\) is a tangent to the curve. Find the value of the constant \(k\).