You mean (x+6)^3, right?
That's (x+6)(x+6)(x+6)
Do two of the (x+6)s first:
(x+6)(x+6)
FOIL
x^2+6x+6x+36
x^2+12x+36
Now the third (x+6)
(x^2+12x+36)(x+6)
In the x+6, the x hits each thing in (x^2+12x+36), then the 6 hits each thing in (x^2+12x+36).
x(x^2+12x+36) + 6(x^2+12x+36)
x^3+12x^2+36x + 6x^2+72x+216
x^3+18x^2+108x+216