Let x be the compression on the spring. As the block is in equilibrium Mg – (kx + rwVg) = 0 where rw 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.,
mvessel + mwater + rwVg
Since the scale has been adjusted to zero without the block, the new reading is rwVg.