$a)$

$A=\left\{1,3,6,7,8\right\}$

$B=\left\{1,2,3,4,5\right\}$

$A\cap B=\left\{1,3\right\}$

 

 

$b)$

$A=\left\{a,b,c,d,e\right\}$

$B=\left\{c,d,e,f,g,h,i\right\}$

$A\cap B=\left\{c,d,e\right\}$