Na in NaCl has eight corner and six face atoms and thus, if we remove face centred atom of one axis, then two face atoms are removed. Thus, A is at 8 corners & at 4 face centers.
Number of A atoms per unit cell $=8\times \frac{1}{8}+4\times \frac{1}{2}=3$
B present in all octahedral voids, hence
Number of B atoms per unit cell $=12\times \frac{1}{4}+1=4$
Thus, formula is ${\mathrm{A}}_3{\mathrm{B}}_4$.