Calculations in physics need SI units: distance in metres (m) and time in seconds (s). Everyday figures often use other units, so you must convert first. 1 km = 1000 m, 1 minute = 60 s and 1 hour = 3600 s.
A rate tells you how much of something happens per unit time. Speed is a rate: speed = distance / time. To change a speed from km/h to m/s, divide by 3.6. For example, 36 km/h is 36000 m in 3600 s, which is 10 m/s. Going the other way, multiply by 3.6, so 30 m/s is 108 km/h.
This is ratio and proportional reasoning: if you double the distance in the same time, the speed doubles. Always write the units at each step and check the answer is sensible.