Calculating correct mean and standard deviation

This question is asking me to calculate the correct mean and standard deviations for data that was over-measured by 3.5cm. The problem is that I only have the sample mean and standard deviation, both calculated from the incorrect measurements (which it does not give). What do I do...? There were 25 measurements taken. I have a feeling that I should calculate the mean and standard deviation using 25 values of "3.5cm" and then subtract the results from the original incorrect values, but this is a total guess.