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#.