3 It is given that \(\cos a = \frac { 3 } { 5 }\), where \(0 ^ { \circ } < a < 90 ^ { \circ }\). Showing your working and without using a calculator to evaluate \(a\),
- find the exact value of \(\sin \left( a - 30 ^ { \circ } \right)\),
- find the exact value of \(\tan 2 a\), and hence find the exact value of \(\tan 3 a\).