The radius of the tires of a car is 18 inches, and they are revolving at the rate of 651 revolutions per minute. how fast is the car traveling in miles per hour?

So the car is moving at 651 revolutions per minute, with wheels of a radius of 18inches

so, one revolution, is just one go-around a circle, and thus 2π, 651 revolutions is just 2π * 651, or 1302π, the wheels are moving at that "angular velocity"

now, what's the linear velocity, namely, the arc covered per minute

well   

now, how much is that in miles/hrs?  well
let's keep in mind that, there are 12inches in 1foot, and 5280ft in 1mile, whilst 60mins in 1hr

thus   

notice, after all the units cancellations, you're only left with mi/hrs