The coefficient of x

(a) ${ }^{18} C_{8}$

Suppose the $(r+1)$ th term in the given expansion contains $x^{8} y^{10}$.

Then, we have

$T_{r+1}={ }^{18} C_{r} x^{18-r} y^{r}$

For the coefficient of $x^{8} y^{10}$ We have

$r=10$

Hence, the required coefficient is ${ }^{18} C_{10}$ or ${ }^{18} C_{8}$