Let x be the compression on the spring. As the block is in equilibrium Mg – (kx + r_{w}Vg) = 0 where r_{w} is the density of water and V is the volume of the block. The reading in the pan is the force applied by the water on the pan i.e.,

m_{vessel} + m_{water} + r_{w}Vg

Since the scale has been adjusted to zero without the block, the new reading is r_{w}Vg.