What is the percentage change in the cost of revenue between 2017 and 2019?
{"Cost of revenue": {"2019": 207, "2018": 78, "2017": 52}, "Selling and marketing": {"2019": 2967, "2018": 2656, "2017": 1577}, "Research and development": {"2019": 2013, "2018": 1801, "2017": 1028}, "General and administrative": {"2019": 4450, "2018": 4415, "2017": 2821}, "Stock-based compensation expense included in expenses": {"2019": 9637, "2018": 8950, "2017": 5478}}
cost_of_revenue_2019 = df["Cost of revenue"]["2019"]
cost_of_revenue_2017 = df["Cost of revenue"]["2017"]
answer = ( cost_of_revenue_2019 - cost_of_revenue_2017 ) / cost_of_revenue_2017 * 100.0