1) F=mv^2/r

The centripetal acceleration is given by v^2/r in the above term (compare to f=ma, a would be everything apart from the m in above formula).

Substitute values in question make sure that v is converted to m/s so i.e. 6km/h divide that by 3.6, giving 6/3.6 m/s (the reason we are dividing by 3.6 is to do with the units. So if you look at 6 km/hr, then if you wanted to express this as m/s we need to change the km to m and the hr to seconds). So if you travel 6km/hr = 6000 m/hr, then 1 hr to seconds = 60*60 = 3600 , therefore 6000/3600 m/s which is essentially 6/3.6 m/s.

2) Once again apply the formula F=mv^2/r