Well
It needs to calculate the volume of a rectangular prism (10*20*20) and subtract from it the volume of a half cylinder with a radius of 10 cm and height of 20
as follows:
prism volume = 10*20*20 = 4000 cm^3
half cylinder volume = 0.5 * pi * r^2 * h = 0.5* 3.14 * 10^2 * 20 = 3140 cm^3
Subtract 3140 from 4000, results in: 860 cm^3
I hope that explains what you are stuck in