What was the change in gross margin between 2018 and 2019?
{"": {"2019": "", "2018": "(dollars in thousands)", "2017": ""}, "Cost of revenue": {"2019": 149215, "2018": 144349, "2017": 142867}, "Gross profit": {"2019": 427308, "2018": 393542, "2017": 339118}, "Gross margin": {"2019": 74.1, "2018": 73.2, "2017": 70.4}}
gross_margin_2019 = df["Gross margin"]["2019"]
gross_margin_2018 = df["Gross margin"]["2018"]
answer = ( gross_margin_2019 - gross_margin_2018 )