(1/7a-7c)^3 = (1/7a-7c)(1/7a-7c)(1/7a-7c)= (1/49a^2 - 2/7ac + 49c^2)(1/7a-7c)= 1/343a^3 - 1/49ac^2 - 14c^3 + 7/49a^2 - 7c + 49ac
(1/7a-7c)^3 = (1/7a-7c)(1/7a-7c)(1/7a-7c)
= (1/49a^2 - 2/7ac + 49c^2)(1/7a-7c)
= 1/343a^3 - 1/49ac^2 - 14c^3 + 7/49a^2 - 7c + 49ac