9. The first three terms of a geometric series are \(( x - 2 ) , ( x + 6 )\) and \(x ^ { 2 }\) respectively.
- Show that \(x\) must be a solution of the equation
$$x ^ { 3 } - 3 x ^ { 2 } - 12 x - 36 = 0$$
- Verify that \(x = 6\) is a solution of equation (I) and show that there are no other real solutions.
Using \(x = 6\),
- find the common ratio of the series,
- find the sum of the first eight terms of the series.