Use the the Binomial Theorem to expand the binomial.

(3a+4x)^3

(j + k)^3 = j^3 + 3 j^2 k + 3 j k^2 + k^3

(3a+4x)^3 = (3a)^3 + 3[(3a)^2 (4x)] + 3 [(3a) (4x)^2] + (4x)^3