What was the change in cost of revenue between 2017 and 2018?
{"": {"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}}
cost_2018 = df["Cost of revenue"]["2018"]
cost_2017 = df["Cost of revenue"]["2017"]
answer = cost_2018 - cost_2017