You have two cylinder of steel, presumably 2.6 m long and 15 mm diameter. Volume of a cylinder of length l and diameter d is pi.l.(d/2)^2 The cuboid is 2.6 x 0.15 x 0.10 cubic m and you have to subtract the volume of the 2 cylinders to find the volume of concrete. Now volume of concrete x density of concrete = mass of concrete. (You're not asking for mass of steel, so density of steel is irrelvant at this point.)