What is the change in the gross margin between 2017 and 2018?
{"": {"2018": "", "2017": "(dollars in thousands)", "$": "", "%": ""}, "General and administrative": {"2018": 31462.0, "2017": 22895.0, "$": 8567.0, "%": 37.4}, "% of revenue": {"2018": 21, "2017": 22, "$": "", "%": ""}}
percent_of_revenue_2018 = df["% of revenue"]["2018"]
percent_of_revenue_2017 = df["% of revenue"]["2017"]
answer = percent_of_revenue_2018 - percent_of_revenue_2017