i cant find out how to work this out??
IF you mean "what is 102.1604043 mm^3 to the nearest cubic mm", not "mm", then the answer is 102 cubic mm., of course. 102.1... is below 102.5 so it is nearer 102 mm than 103 mm.
If you mean to take the cube root of 102.1604043 and then ask "what is it to the nearest mm.", it is easy to use a calculator to see that which is larger than 4.5 mm and so is 5 mm, to the nearest mm.