How do you write 216 as a power of 6?

You want to solve 6x = 216. For such a simple problem (since 216 is an integer power of 6), you can use trial and error, that is to say, try out different numbers for "x". For a more general method, that works in other cases, you can calculate this as log(216) / log(6). Just make sure you use the same base (either base 10 or base e) for both logarithms).