How do you solve this physics problem?

This is mostly a problem of unit conversion. What you need to know is that there are 1000 meters in a kilometer and 3600 seconds in an hour. So now 100km/hr becomes 100*1000/3600 m/sec and 80km/hr is just 4/5 of whatever the cheetah's speed is. Take the difference in speeds and you find out how quickly the cheetah is closing the gap. Something like 20*1000/3600 m/sec. Then you just divide your 70m by this 20*1000/3600 m/sec speed and you should get your time.