Chapter 7 - Exponents and Exponential Functions - 7-2 Scientific Notation - Practice and Problem-Solving Exercises - Page 424: 35

$6\times10^{-10}$ $8\times10^{-8}$ $9\times10^{-7}$ $7\times10^{-6}$

You can order a list of numbers in scientific notation from least to greatest by looking at their exponent where the smallest exponent would translate the smallest number. Therefore you would order your list accordingly as shown in the answer.