A block can slide down an a smooth inclined plane of inclination $\theta$ , kept on a floor of a lift. When the lift is descending with a retardation a. The acceleration of the block relative to the incline is

The lift descending with a retardation, that implies the net acceleration of the lift is downwards.

If we observe w.r.t lift there will be a pseudo force ma directed downwards.

So the effective acceleration due to gravity will be g+a.

So the acceleration of the block relative to the incline is $(\mathrm{g}+\mathrm{a})\mathrm{sin\theta }$