All the surfaces shown in figures are assumed to be frictionless. The block of mass m slides on the prism which in turn slides backward on the horizontal surface. Find the acceleration of the smaller block with respect to the prism.

Let the acceleration of the prism be a0 in the backward direction. Consider the motion of the smaller block from the frame of the prism.
The forces on the block are
(i) N normal force,
(ii) mg downward (gravity)
(iii) ma0 forward (pseudo)