What was the change in the transaction fees between 2017 and 2019?
{"Merchant fees": {"2019": 361755, "2018": 297776, "2017": 234548}, "Interchange fees": {"2019": 44150, "2018": 51128, "2017": 44410}, "Transaction fees": {"2019": 405905, "2018": 348904, "2017": 278958}, "Servicing fees(1)": {"2019": 123697, "2018": 65597, "2017": 46575}, "Other(2)": {"2019": 44, "2018": 172, "2017": 354}, "Servicing and other": {"2019": 123741, "2018": 65769, "2017": 46929}, "Total revenue": {"2019": 529646, "2018": 414673, "2017": 325887}}
transaction_fees_2019 = df["Transaction fees"]["2019"]
transaction_fees_2017 = df["Transaction fees"]["2017"]
answer = transaction_fees_2019 - transaction_fees_2017