You have an equation

a)

where R the reading of the richter scale, I is the intensity of the earthquake, the base intensity.

You want to find R. You have I.

b) Now you want to find I. You have R.

You will be able to remove the log function by applying the exponential function to both sides (assuming the log = ln. Otherwise, use the base of the log instead of e)

EDIT: I thought about it for a while. The richter scale's logarithm is in base 10 because an increase of 1 on the scale is a tenfold increase in intensity. So