Which is a sum of cubes?

a3 + 18

a6 + 9

a9 + 16

a12 + 8

Ok so

a cube is x^3 or x^ (3m) where m is a natural (counting) number

look at each, if one is not a cube, it is not correct

a^3+18

18 is not a perfect cube

a^6+9

9 is a square, not cube

a^9+16

16 is 2^4 or 4^2, not a perfect cube

a^12+8

a^12 = (a^4) ^3

8=2^3

(a^4) ^3 + (2) ^3

answer is last one