Answer
$r_{A}=\frac{d_{1}}{2}$.
$r_X=d_2-\frac{d_1}{2}$
Work Step by Step
The distance of a bond between 2 atoms is the sum of the atomic radii of the 2 atoms.
The atomic radii of A is is therefore $\frac{d_{1}}{2}$.
The distance of the bond between A and X, $d_2$, is the sum of the atomic radii of A and X.
$d_2=r_A+r_X$
$d_2=\frac{d_1}{2}+r_X$
$r_X=d_2-\frac{d_1}{2}$
