When can I hear the babies heartbeat?

Last week we went for out first ultrasound at what I thought was 6w5d but the ultrasound said I was only 5w2d. Felt kinda sad because we couldn't hear the babies heart. Doctor told me to come the next week (this week) if I was feeling anxious and wanted to hear the hearbeat, but I would prefer to go when FOR SURE we'll hear the babies heart. When could that be!?