One man starts from a city and walks north at the rate of 3 kilometers per hour. Three hours later,a second man starts the same route at 4 kilometers per hour. How far will he have to go before he overtakes the first man?

You can do this in two ways, do it in one way and check it using the second way.

The first is to use the speed/distance/time formula and the second is to use a velocity-time graph and use the areas.

Can you post your attempt? :)