Fa , Fb = resultant forces of bending stresses acting on the bottom 2 x 12 board
Since Fb > Fa (due to the ΔM difference in moment magnitudes at a and b), the 2 x 12 board does not satisfy equilibrium.
When M and M + ΔM are applied at a and b, respectively, the 2 x 12 board will slide in the axial direction (that is, the x direction) ...