$\mathrm{A}$forms ccp structure that means number of $\mathrm{A}$ atoms per unit cell $\to 4$

$\mathrm{B}$ atoms per unit cell $\to 4$ (Octahedral voids), as number of octahedral voids will be equal to number of atoms per unit cell.

$\mathrm{A}$ removed $=2\times \frac{1}{2}=1$

$\mathrm{A}$ left $=4-1=3$

Formula becomes ${\mathrm{A}}_3{\mathrm{B}}_4$

$\mathrm{x}=3$.