There is a process called multiplication. To undo it, one uses division.
There is a process called exponentiation. To undo it, one takes a logarithm.
If a = b^c, then c = log_b(a) (log base b of a)
e.g. 64 = 2^6, so 6 = log_2(64)
natural logarithm (base e) is written ln
logarithm base 10 is written log
logarithm base 2 is written lg