In a solid AB, A atoms are in ccp arrangement and B atoms occupy...

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

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

$A$ removed $= 2 \times \dfrac{1}{2} = 1$

$A$ left $= 4 - 1 = 3$

Formula becomes ${A}_{3}{B}_{4}$

$x = 3$.