take it in steps. to get from km to m jus multiply by 1000 then to get from hours to sec divide by 60 to get min and then by 60 again to get sec --- mutiply by 1000 and divide by 3600 Another PF thread on this topic is here https://www.physicsforums.com/threads/how-to-convert-km-h-to-m-s-squared.90825/
You want a method of converting that is easy to use in all situations. The one I like goes like this: 100 km/h = 100 * 1000 m/3600 s = .... You just replace the km with 1000m, and the hour with 3600 s. This method always works and soon you don't even have to think about it. You can even do km/hr/minute or something weird like that.