Use Method of Section:
1. Cut the structure in half so that the 'cut' is through the members that you are looking for (i.e. top to bottom cut, through three members: top, a, and b).
2. Guestimate and put symbols on members a and b (and top if you want) if they are either in tension or compression.
3. Calculate moments around a joint (do joint at centre of truss where the 390 N is pointing). This should solve member force in b, and tell you if in compression or tension.
4. Calculate moments around joint directly left to of first joint calculated (on other side of 'top' member that you cut). This should solve member force in a.
