A logarithm is kind of like reverse exponents. There is a base and a number in a logarithm. The point of a logarithm is to find out what power you have to raise the base to get the number next to the base. For example:
log base 5 of 25 = x
This is the same thing as saying 5 to the xth power is 25, which is 2 (so x would be 2). Using this example, your function must take the 5 and 25 and somehow get 2.
logarithm(5, 25) ➞ 2
logarithm(2, 64) ➞ 6
logarithm(2, 4) ➞ 2
"Invalid" for inputs outside of domain.