The answer is 16b.
First, look at the problem 9a^2 - 24ab + ____.
Factor it out while keeping in mind the missing constant.
Then, think about a number that when inserted into the blanks, will make the statement (3a)(?) + (3a)(?) = 24ab.
Let try 4b. 
(3a - 4b)(3a - 4b)
Use the foil method to make sure the statement is correct.
=9a^2 - 12ab -12ab +16b.
=9a^2 - 24ab + 16b.

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