Inactive Tutor answered 02/20/20
The binomial theorem gives the terms as (9Ck)•xk•(-2/x2)9-k for k=0,1,...,9. We can rewrite this as (9Ck)(-2)9-k•x3k-18. The constant term is the same as the coefficient on the zero-degree term, which occurs when k=6. The coefficient on it is (9C6)(-2)3 = -672.