Explanation:
72 hours total baking during 6 days.
that means 72/6 = 12 hours of baking every day.
3 hours of these 12 hours he used to bake bread.
that leaves 12 - 3 = 9 hours every day for baking cakes.
so, to summarize all this in an arithmetic expression :
x = 72/6 - 3 = 9