You would want to multiply by a series of conversion factors.
One of the easiest methods is to figure out the units, then deal with the quantities.
To convert kilometers to meters we know that there are 1000 meters in every kilometer, so we multiply kilometers by (1000m)/km. Note that the km "cancel" (divide to 1) leaving m as the units.
To convert hours to seconds we can either convert hours to minutes and then minutes to seconds, or use the fact that there are 3600 seconds in an hour. So we multiply 1/hr by (1hr)/(3600s); the hour units cancel leaving the seconds units in the denominator.
Putting this together yields km/hr=km/hr * (1000m)/km * (hr)/(3600s)
Reducing the fractions leaves multiplying by 5/18
For example, 50km/hr is equivalent to 125m/9s or 13.888m/s
Step by step:
`1(km)/h * 1000 m/(km) * (1h)/(3600s) = 1/3.6 (m/s)`
Divide km/h by 3.6 to get m/s.