Assuming:-
- your context is set up correctly and you have all the navigation properties in place
- your query is "Get me all of the purcahse orders created by any employee who shares a role with employee #17"
You can use:-
context.Employees.Where(x => x.Id == 17)
.SelectMany(x => x.Roles)
.SelectMany(x => x.Employees)
.Distinct()
.SelectMany(x => x.PurchaseOrders);