Is there a picture of the data set? I cannot give you an exact answer without the actual data values, but I can explain how to solve it.
The mean absolute deviation basically tells the average of how much each data value deviates from the mean of the entire data set. Therefore you just find the difference between each value in the data set and 57. Then you take all the differences and find the average by adding them all up and dividing by the number of values.
