6 The line \(l _ { 1 }\) has equation \(\mathbf { r } = \left( \begin{array} { r } 3
0
- 2 \end{array} \right) + s \left( \begin{array} { r } 2
3
- 4 \end{array} \right)\). The line \(l _ { 2 }\) has equation \(\mathbf { r } = \left( \begin{array} { l } 5
3
2 \end{array} \right) + t \left( \begin{array} { r } 0
1
- 2 \end{array} \right)\).

1. Find the acute angle between \(l _ { 1 }\) and \(l _ { 2 }\).
2. Show that \(l _ { 1 }\) and \(l _ { 2 }\) are skew.
3. One of the numbers in the equation of line \(l _ { 1 }\) is changed so that the equation becomes \(\mathbf { r } = \left( \begin{array} { l } 3
   0
   a \end{array} \right) + s \left( \begin{array} { r } 2
   3
   - 4 \end{array} \right)\). Given that \(l _ { 1 }\) and \(l _ { 2 }\) now intersect, find \(a\).