Express the quantity 556.2 x 10^-12 in each units
a.) ms
b.) ns
c.) ps
d.) fs

The
we should know the meaning of each abbreviation:
ms means  millisecond, its value is 10^-3 s 
ns means  means  nanosecond,   its value is 10^-9 s
ps means  picosecond, its value is 10^-12 s
fs means  femtosecond, its value is 1x 10^15 s

Expressions of the quantity 556.2 x 10^-12 are
556.2 x 10^-12 =556.2 ps
556.2 x 10^-12 =556.2 x 10^-9 x 10^-3= 556.2 x 10^-9 ms
556.2 x 10^-12 = 556.2 x 10^-3 x 10^-9 = 556.2 x 10^-3 ns
556.2 x 10^-12 = 556.2 x 10^- 27 x 10^15 = 556.2 x 10^- 27 fs