$(1-i)^8 = ((1-i)^2)^4 =(1-2i+i^2)^4=(-2i)^4=16i^4=16$
$(\dfrac{1+i}{1-i})^{21} = (\dfrac{(1+i)^2}{1-i^2})^{21}=(\dfrac{2i}{2})^{21}=i^{21} \Rightarrow (i^{13}-\dfrac{1}{i^{13}}).i^{21}$
$=i^{34} -i^8=(i^2)^{17} -(i^2)^4=-1-1=-2$
Vậy $z= 16-2=14$ vậy $|z| = ...$ khó quá oáp oáp =))