add math 困難問題,請高手指點~~

find the constant term in the expansion of (2x^2+1/x)^9
我知個答案係
=9Cr(2x^2)^9-r(1/x)^r
=9Cr ．2^9-r．x^18-3r
18-3r=0
r=6
個term係672
但我唔知條式第二行2^9-r ．x^18-3r 點離
help me~~~