= Solution
For reflection $222$, the positional phase factor is $e^{2\pi i(2x+2y+2z)}=1$ for each of the four sites just found. Treat the supplied anomalous increment as the imaginary term $if''$, with $f''=4.8$ electrons per Se atom. Then
$$
\Delta F(222)=4i(4.8)=19.2i\ {\rm electrons}.
$$
Its magnitude is \b[$19.2$ electrons] and its phase is \b[$+90^\circ$] relative to the real axis in this structure-factor convention. Translating the origin rotates both this reference contribution and the ordinary <structure factor> consistently; the physically relevant phase difference is invariant.
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