Answer:
2.0 * 10^-6
Explanation:
Because both numbers have the same scientific notation, you can just subtract the 9.1 from the 8.9 and keep the scientific notation.
(9.1*10^-6) - (8.9*10^-6)
(9.1 - 8.9)*10^-6
(2)*10^-6
Now that you have the answer, you need to use scientific notation, which is
2*10^-6, or 2.0*10^-6.
If anything is unclear, please let me know!