Supply the volume of the insoluble substance from the displaced liquid.

displaced liquid: 1.724 l

volume: ? cm^3

can someone explain to me how to do this .

The volume of the insoluble substance is 1724mL which equals 1724cm³.  We know this since the displacement of water is how much it rose from its original height which had to have come from the substance added.  I am going to scale down your example so that I can explain it in a lab setting.  Say you are tasked with finding the volume of a given insoluble substance with only a graduated cylinder and water.  first you need to fill up the graduated cylinder with enough water so that you know the volume of water is greater than what the volume of the substance.  lets say you decide on 3.00mL and when you added the substance the volume in the graduated cylinder increased to 4.724mL.  This is telling us that the displacement of water was 1.724 mL which means the volume inside the graduated cylinder increased by 1.724mL.  The increase in volume could not have come from anything but the substance added so we can say the substance has a volume of 1.724. (1.724mL substance +3.00mL water=4.724mL total)
1mL=1cm³
I hope this helps.

Ithink the answer is 56.