The main thing here is the bit where you rename the table to something shorter: cd.members > mem. However this site clearly doesn't do that.
The concept is simple enough, you just have to join two tables that are related via a foreign key. When you join them together, you get a more detailed row.
select bks.starttime from cd.members mem inner join cd.bookings bks on mem.memid = bks.memid where mem.firstname = 'David' and mem.surname = 'Farrell';