The accompanying data file contains 57 months of tax revenue from medical and retail marijuana tax and fee collections. For cross-validation, let the training and the validation sets comprise the first 45 months and the last 12 months, respectively. 

a. Use the training set to implement the Holt exponential smoothing method with user-supplied (α = 0.30 and β = 0.20), as well as the computer-generated, smoothing parameters and compute the resulting MSE, MAD, and MAPE for the validation set. 

b. Determine the preferred model and reimplement it with the entire data set to forecast tax revenue for the 58th month