In this problem the terms have different powers, and so can not be subtracted directly. Instead one term must be converted

5 x 10^-5 - 5 x 10^-9 = 5 x 10^-5 - 0.0005 x 10^-5 = (5 - 0.0005) x 10^-5 = 4.9995 x 10^-5