Effective buckling length of a steel angle connected by double rivets is
Following table gives the effective buckling length of different types of angles struts
Type 
End connection 
Effective length (l) 
1. Single angle discontinuous 
i. One rivet or bolt at each end 
l = L 
ii. Two or more rivet or bolts at each end 
l = 0.85 L 

2. Double angle, discontinuous, stitched together 
i. Connected on the same side of gusset plate a) One rivet or bolt at each end b) Two or more rivet or bolt at each end 
l = L
l = 0.85 L 
ii. Connected on both side of gusset plate by two or more rivet or bolt 
l = 0.7 to 0.85 L 

3. Single or double angle, continuous 
One or more rivet or bolt 
l = 0.7 to 0.85 L 
Important point:
In question it is not mentioned whether it is for Single angle or double angle, but in both the cases for two or more rivets (double rivets) the effective buckling length is 0.85 L.