CREATE TABLE table_73986 ("Name" TEXT, "Park" TEXT, "Length  ( ft ) " FLOAT, "Height  ( ft ) " FLOAT, "Speed  ( mph ) " TEXT, "Opened" TEXT, "Status" TEXT)

What is the length of the coaster with the unknown speed
SELECT "Length (ft)" FROM table_73986 WHERE "Speed (mph)" = 'unknown'
SELECT "Length (ft)" FROM "table_73986" WHERE "Speed (mph)" = 'unknown'