9 The position vectors of \(A , B\) and \(C\) relative to an origin \(O\) are given by
$$\overrightarrow { O A } = \left( \begin{array} { r }
2
3
- 4
\end{array} \right) , \quad \overrightarrow { O B } = \left( \begin{array} { c }
1
5
p
\end{array} \right) \quad \text { and } \quad \overrightarrow { O C } = \left( \begin{array} { l }
5
0
2
\end{array} \right) ,$$
where \(p\) is a constant.
- Find the value of \(p\) for which the lengths of \(A B\) and \(C B\) are equal.
- For the case where \(p = 1\), use a scalar product to find angle \(A B C\).