Assignment solutions are displayed for pupils within the same Classwork modal that they used to access the assignment and submit their work. The solution becomes available at the point defined by the teacher when creating the assignment — immediately, following submission, or at a particular date.
The easiest way for your pupils to get to this would be for them to go to the the appropriate lesson on their Timetable page. They will find their Classwork in the lesson summary on the left-hand side.
Instead of going via the timetable, they can also find the Classwork and solutions via the Assignments page. By selecting the Submitted tab, they will find a collection of their completed assignments — classwork, homework & assessments.
Clicking Open on the classwork in question, following either of the above methods, pupils will then see a Solutions option above the record of their submission.