$a)D_f=\left\{10,11,12,13,14,15,16,17,18,19,20\right\}$

$Z{W}_f=\left\{2,1,0,-1,-2,-3,-4,-5,-6,-7,-8\right\}$

 

$b)x=11$    

$c)$  

 

$x$	$10$	$11$	$12$	$13$	$14$	$15$	$16$	$17$	$18$	$19$	$20$
$f\left(x\right)$	$1-0=1$	$1-1=0$	$-1$	$-2$	$-3$	$-4$	$-5$	$-6$	$-7$	$-8$	$2$