$\text{ Statement }1:\text{ If }B=U-A\text{ then }n\left(B\right)=n\left(\mathrm{U}\right)-n\left(A\right)\text{ where }U\text{ is universal set. }$

$\text{ Statement }2:\text{ For any three arbitrary sets }A,B,C\text{ we have if }C=A-B\text{ then }n\left(C\right)=n\left(A\right)-n\left(B\right)\text{ . }$

$\text{ If }U\text{ is universal set then }B=U-A=A^{\mathrm{\prime }},\text{ for which }n\left(B\right)=n\left(A^{\mathrm{\prime }}\right)=n\left(\mathrm{U}\right)-n\left(A\right)$

$\text{ But for any three arbitrary sets }A,B,C\text{ we cannot have always }n\left(C\right)=n\left(A\right)-n\left(B\right)\text{ if }C=A-B\text{ as it is not specified here that }$

$\text{ weather }A\text{ is universal set or not. In case }A\text{ is not universal set we cannot conclude that }n\left(C\right)=n\left(A\right)-n\left(B\right)\text{ . }$

Hence, statement 1 is true but statement 2 is false.