Megameter means a million(10^6) meters, or 1000 kilometers. It would be too large to express the size of a bacterial cell.
The following answer is under the assumption that the volume is 2.56 cubic color(red)"micrometers". If you really want the volume of a gigantic bacterial cell, see SCooke's post.
"Micro" means one-millionth(10^-6).
1 μm=10^-6 m
1 μm^3 =(10^-6)^3 m^3= 10^-18 m^3
The volume of the bacterial cell is 2.56xx10^-18 m^3.
(a) 1 mm^3 = (10^-3)^3 m^3 =10^-9 m^3
2.56*10^-18 m^3 = (2.56xx10^-18)/(10^-9) mm^3 = 2.56xx10^-9 mm^3.
(b) The volume of 10^5 cell is 2.56xx10^-18 *10^5=2.56xx10^-13 m^3.
As 1 liter is 10^-3 cubic meters, the volume is
(2.56xx10^-13)/(10^-3) L=2.56xx10^-10 L.