3^4=81. how do you write as a logarithm

The form it is currently in is called the exponential form. Which is represented with this equation: 

y = b^x

The equivalent logarithmic equation is: 
x = log_b y

Where b, is a base.
So our power becomes what the problem is equal to, the number that is being brought to that power is the base. The one on the equal sign is what we are taking the log of. So it will look like this:

4 = log_3 81