All surfaces are smooth. The acceleration of mass m relative to wedge is

W.r.t the frame of the incline, there will be a pseudo force acting on the block in horizontal direction.

When it is divided into two components, $ma\cos \theta$ along the incline and $ma\sin \theta$ perpendicular to the plane.

So the resultant acceleration of the block will be $g\sin \theta +a\cos \theta$