Questions that involve a change of base are quite rare but it is still possible that they will come up in your exam. It should be obvious that a change of base is required because the base is 3 on one of the log expressions whereas it is x on the other. The change of base formula is given (in the Edexel formula booklet) as:
Choosing a=x, x=81 and b=3 in this formula gives:
Substituting this into the original expression gives:
The final log expression can be evaluated as the power of 3 that gives 81 and so the final answer is 4.