In active voice, a sentence is structured as 'someone did an action to them'.
In passive voice, a sentence is structured as 'they had an action done to them by someone'.
Keep that in mind and look at your sentence.
The police (someone) will arrest (did an action) the robbers (them).
Rewrite that using the passive structure and changing the grammar as you go.
The robbers will be arrested by the police.
