Determine the angle $\mathrm{\theta }$ between the surface which is perpendicular to the axial force $\mathrm{P}$ (i.e., the horizontal plane) and the inclined plane surface $\mathrm{a}-\mathrm{a}$.

From the geometry of the triangle formed by the inclined surface:

$\mathrm{\theta }={180}^{\circ }-{90}^{\circ }-{36}^{\circ }={54}^{\circ }$