Draw the FBD in that condition:
kx upwards, mg of block + m'g of weight downwards and B buoyant force upwards.
kx = 50*(3*10-2) = 1.5 N upwards
mg of block = VÏg = (3*10-2)3*800*10 = 0.216N downward
m'g = m' is to be calculated
B = VÏwg = (3*10-2)3*1000*10 = 0.27N upward
Considering equilibrium,
1.5-0.216-10m'+0.27=0
or m' = 0.1554 kg
Calculation mistakes possible