I don't understand how to do this...
topsquark noticed that 9a - 7a = 2a. how did he do that? well, he could have done it by experience, but there is a method you can do.
take the coefficient of a^2, that is 3 and multiply it by the constant term, that is, -21. the result is -63. now you want two numbers that when multiplied give -63 but when added give 2 (which is the coefficient of the middle term). 9 and -7 are such numbers. now we split the middle term into those two numbers and continue as topsquark did