Which relation is a function?

Find the amount in a continuously compounded account for the given condition. principal: $1000, annual interest rate: 4.8%, time: 2 yr

Acar started driving from city a to city b, distance between which is 620 miles, at a rate of 60 mph. two hours later a truck left city b and started moving towards city a at a rate of 40 mph. how many hours after the truck's departure will the two meet?

What is the effect on the graph of the function f(x) = x2 when f(x) is changed to f(x) − 4?