I'm not sure what exam board you're on, so I don't know where to start to look. However, I can explain how hanging valleys are formed:
1. A primary glacial valley has tributaries attached to it.
2. The primary valley has thicker ice, so it has more erosive power compared to the tributary valley.
3. The valley floor of the primary valley in turn eroded at a higher rate through abrasion and plucking.
4. Thus the primary valley is deeper than the tributary valley.
5. During deglaciation of an interglacial period, the glaciers will ablate to leave behind the valleys exposed.
6. The tributary valley 'hangs' over the primary valley because it is less deep.
7. If the hanging valley contains meltwater or a new river, it may create a waterfall at the confluence of the two valleys because the primary valley is deeper.
Hope this helps.