Answer:
8.  272 m²
10. 306.9 km²
12. 311.4 m²
Explanation:
8. the surface area = 
 (10×10)+(4×½×10× 8.6)
 = 100 + 172 = 272 m²
10. the surface area =
 (11×7)+(2×½×7×13.2)+(2×½×11×12.5)
 = 77+92.4+137.5= 306.9 km²
12. the surface area =
 (9×9) +(4×½×9×12.8) 
 = 81+230.4 = 311.4 m²