It is important to understand the situation.
The last line states that the reflected sound from the 8m wall reflects from the 2m wall and is then detected by the microphone.
So the waves cover the entire width of 10m and the extra 2m.
So total distance covered by sound waves = 10 + 2 = 12m
Time = 0.010s
Speed = Distance ÷ Time = 12 ÷ 0.01 = 1200 m/s