What is the constant term of the expansion of $\left(6x+\dfrac{1}{3x}\right)^6$?
expand (6 x + 1/(3 x))^2.............................................(1)
[36 x^2 + 1/(9 x^2) + 4] * (6 x + 1/(3x))..........................(2)
[216 x^3 + 1/(27 x^3) + 36 x + 2/x] * (6 x + 1/(3x))..............(3)
[1296 x^4 + 1/(81 x^4) + 288 x^2 + 8/(9 x^2) + 24] * (6 x + 1/(3 x))........(4)
[7776 x^5 + 1/(243 x^5) + 2160 x^3 + 10/(27 x^3) + 240 x + 40/(3 x)] * (6 x + 1/(3 x))....(5)
46656 x^6 + 1/(729 x^6) + 15552 x^4 + 4/(27 x^4) + 2160 x^2 + 20/(3 x^2) + 160...............(6)