What was the percentage change in cost of revenue from 2017 to 2019?
{"": {"2019": "", "2018": "(In thousands, except percentage data)", "2017": ""}, "Cost of revenue": {"2019": 704535, "2018": 717118, "2017": 731453}, "Gross margin percentage": {"2019": 29.5, "2018": 32.3, "2017": 29.6}}
cost_2019 = df["Cost of revenue"]["2019"]
cost_2017 = df["Cost of revenue"]["2017"]
answer = (cost_2019 - cost_2017) / cost_2017 * 100.0