What was the difference in net cash provided by operating activities between 2017 and 2018?
{"Net cash provided by (used in): -- Operating activities": {"2019": -156832, "2018": 113207, "2017": 64241}, "Net cash provided by (used in): -- Investing activities": {"2019": 25761, "2018": 17496, "2017": -28718}, "Net cash provided by (used in): -- Financing activities": {"2019": -8406, "2018": 1287, "2017": 4635}, "Net cash provided by (used in): -- Net change in cash and cash equivalents": {"2019": -139477, "2018": 131990, "2017": 40158}}
net_cash_2018 = df["Net cash provided by (used in): -- Operating activities"]["2018"]
net_cash_2017 = df["Net cash provided by (used in): -- Operating activities"]["2017"]
answer = net_cash_2018 - net_cash_2017