(a) Accelerations of particle and block are shown in figure.Acceleration of particle with respect to block= acceleration of particle − acceleration of block =(gsinθi^+gcosθj^​)−(gsinθ)i^=gcosθj^​​Now, motion of particle with respect to block will be a projectile as shown.The only difference is, g will be replaced by gcosθ⇒​∴PQ=Range(R)=gcosθu2sin2α​PQ=gcosθu2sin2α​​