Computable Scott sentences and the weak Whitehead problem for finitely presented groups

We prove that if $A$ is a computable Hopfian finitely presented structure, then $A$ has a computable $d$-${\mathrm{\Sigma }}_2$ Scott sentence if and only if the weak Whitehead problem for $A$ is decidable. We use this to infer that every hyperbolic group as well as any polycyclic-by-finite group has a computable $d$-${\mathrm{\Sigma }}_2$ Scott sentence, thus covering two main classes of finitely presented groups. Our proof also implies that every weakly Hopfian finitely presented group is strongly defined by its ${\mathrm{\exists }}^{+}$-types, a question which arose in a different context.