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(e^x) = x. To solve e^x = 6, take ln of both sides: x = ln(6), about 1.79. Keep the answer in its exact form; a calculator's ln key gives the decimal when you need one.