V = 16cm*35cm*56cm = 31360 cm3
m = ρV (where ρ is density; for water that is 1 g/ml or 1000 kg/m3)
Since cm3 are ml for water, just plug in 1 g/ml and 31360 ml into the equation.
To find kg multiply previous answer by 1 kg/ 1000g
Please consider a tutor.