Answer:
6: We already know the diamer, which is 18, and the radius which is half of 18. So 18 gets multiplied by PI (In this case 3.14) 3.14*18=56.52, or 56.5. Then we divide it by 2 (Cause it's a half circle.) which is 28.25.
7: 20*3.14=62.8/2=31.4
8: 8*3.14=25.12/2=12.56
(Because this time we didn't know the diameter, but 4+4=8.)
Explain: