Exercise 2: Two Steps, Each Depending on Multiple Earlier Chapters — Possible Solution
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EXAMPLE PAIR: STEP 1 (SCANNING)
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Scanning a single item genuinely depends on two separate chapters working together: Chapter 4 supplies the client-side decoding hook that turns a camera image into a barcode string, with no knowledge of any backend at all, and Chapter 3 supplies the actual lookup route and barcode_cache table that the scanning hook's own callback calls once a barcode is decoded. Remove either one and the step breaks - decoding with nowhere to send the result, or a lookup route with no scanner ever triggering it.

EXAMPLE PAIR: STEP 7 (RECIPE SUGGESTION)
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The recipe suggestion step depends on Chapter 6 to identify which items are actually expiring soon (the input list the recipe route queries for), and on Chapter 9 to perform the actual TheMealDB fan-out, caching, and relevance sort. Chapter 9's own route has nothing meaningful to search for without Chapter 6's own alerts query first identifying which real ingredients are running low.

WHY THIS WORKS AS AN ANSWER
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It picks two genuinely separate steps and, for each one, names at least two specific earlier chapters whose individual contributions are both necessary - explaining concretely what would break if either contribution were missing, rather than just listing chapter numbers without justification.