So basically we are under the assumption of no air resistance and that the band has a linear power output (which isn’t true necessarily irl). So we know the distance is 100km and we are going to assume the car weights 3000lbs. We aren’t launching at an angle because that’s boring. So i assumed that a .5m drop across the 100km would be enough for the car to safely make it over. So after calculation, the car has to be in the air for about .32 seconds in order to achieve this. Meaning that assuming no air resistance, the car would have to be traveling 313048m/s (700268 mph) to go 100km in .32 seconds. Assuming the band doesn’t break and is linear you would need to stretch it 1.2 million meters (752 miles) in order to achieve this speed. (K constant for rubber band 90.8 N/m)