Base e

One base shows up so often that it has its own letter: e, a fixed number about 2.718. Like π its decimals never end, and like π it is a constant, not an unknown. It appears whenever something grows continuously, every instant instead of once a year.

The log base e has its own name too: the natural log, written ln. Like any log it undoes its own power: $\ln\left({e}^{x}\right) = x$. To solve ${e}^{x} = 6$, take ln of both sides: $x = \ln\left(6\right)$, about 1.79. Keep the answer in its exact form; a calculator's ln key gives the decimal when you need one.