What is the change in the % of revenue between 2017 and 2018?
{"": {"2018": "", "2017": "(dollars in thousands)", "$": "", "%": ""}, "Other expense, net": {"2018": 4628.0, "2017": 302.0, "$": 4326.0, "%": 1432.5}, "% of revenue": {"2018": 3, "2017": 0, "$": "", "%": ""}}
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