Expand Binomial Theorem I have problem in Binomial Theorem
Question:

Find coefficient of $x^0$ term
  $$[9x-\frac{b}{c\cdot x^2}]^{17}$$


Notice, Binomial expansion $$\left(9x-\frac{b}{cx^2}\right)^{17}$$  $$=^{17}C_{0}(9x)^{17}\left(\frac{b }{cx^2}\right)^{0}+^{17}C_{1}(9x)^{16}\left(\frac{b }{cx^2}\right)^{1}+^{17}C_{2}(9x)^{15}\left(\frac{b }{cx^2}\right)^{2}+\ldots + ^{17}C_{17}(9x)^{0}\left(\frac{b }{cx^2}\right)^{17}$$
$r^{th}$ term is given as $$T_r=^{17}C_{r-1}(9x)^{17-r+1}\left(\frac{b }{cx^2}\right)^{r-1}$$$$=^{17}C_{r-1}(9x)^{17-r+1}\left(\frac{b }{cx^2}\right)^{r-1}$$ $$=^{17}C_{r-1}\left(\frac{b }{c}\right)^{r-1}(x)^{18-r-2r+2}$$
$$=^{17}C_{r-1}\left(\frac{b }{c}\right)^{r-1}(x)^{20-3r}$$
For $x^0$ term, the power of $x$ in $T_r$ should be zero hence let's put $$20-3r=0\iff r=\frac{20}{3}$$ but $r$ is an integer hence $x^0$ term (constant)  is not there in the expansion. 
