Firstly let us get the time down to minutes from hours. Then the calculation will become easy.
1 hour and 45 minutes = (60 + 45) minutes
= 105 minutes
So
The distance traveled by the car in 105 minutes = 98 miles
Then
The distance traveled by the car in 60 minutes = (98/105) * 60 miles
= 5880/105 miles
= 56 miles
So the average speed of the car as can be seen from the above deduction is 56 miles per hour.