You need to learn about Swahili time. First you have to get used to the idea that the day begins at sunrise, not in the middle of the night. That's perfectly logical, especially if you live near the equator, where the sun always rises at what we call 6 am and sets 12 hours later. So you could think of sunrise as 'hour 0' of the new day. When one hour of daylight has passed (at our 7 am) it is 'hour 1', "saa moja" and after two hours (at our 8 am) it is 'hour 2', "saa mbili".
See this helpful video lesson that counts you through the whole day:
Swahili time uses a 12-hour clock, so when you get to sunset at hour 12, "saa kumi na mbili" (or 6 pm to us), you start again at hour 1, which takes you through the 12 hours of darkness. In this question, "saa saba mchana" (afternoon) specifies that you mean 7 hours after sunrise, which is our 1 pm. If it was 7 hours after sunset, you would say "saa saba usiku" (night), which is our 1 am.
NOTE: They don't actually say 'hour 0' for 6 am. They actually say "saa kumi na mbili alfajiri" ('hour 12 dawn'). I just find it easier to think of it as hour 0, probably because I live in a place that uses the 24-hour clock, where midnight is represented as 00:00.
There are lots of ways to get your head around Swahili time. A quick fix if you use the 12-hour clock is just to add or subtract 6 hours from the given Swahili time. (So for saa saba, 7 - 6 = 1 o'clock.)