\(\frac{6a^3}{b}:\frac{ab^2}{12}\qquad \qquad b\ne0\\ \frac{6a^3\div a}{b}:\frac{ab^2\div a}{12}\\ \frac{6a^2}{b}\times 12b:\frac{b^2}{12}\times 12b\\ 72a^2:b^3\\ \)
You can continue to rearrange this. It depends on what form you want it in.
Edit: I have corrected a small error.