Help with Shear Stress Problem

[akmek2010]

i solved the problem but when i solve with finite element (Ansys), only XZ plane shear stress is different. Program results show that 750 N force creates XZ plane shear stress at point b. I think it must be zero for a,b and c points because, moment of area (Q) is zero. Other results are same with my results. I want to know how to calculate that shear stress .