James is about to tackle an orienteering course. He has been given these instructions:
² the course is triangular and starts and finishes at S
² the first checkpoint A is in the direction 056o from S
² the second checkpoint B is in the direction 146o from A
² the distance from A to B is twice the distance from S to A
² the distance from B to S is 2.6 km.

Find the length of the orienteering course