Since you told us the speed limit was 40 miles per hour, we know that we can apply the following conversions:
100cm = 1m
1m = 0.000621371
b) 18.8m/s = 0.01168178miles per second
0.01168178 x 60 x 60 = 42.054408
Therefore, the car is travelling 2.054408 miles per hour above the speed limit.
c) 1750cm/s = 17.5m/s
17.5m/s = 0.010874miles per second
0.010874mps x 60 x 60 = 39.1464
Therefore, the car is travelling 0.8536 miles per hour below the speed limit.
Hope I helped!