First we will find the circumference of the tire.

We know that the circumference of a circle is given by :

C = 2*pi*r such that r is the radius.

==> C = 2pi *21 = 42pi inches.

The tire makes 7 rotations every second.

==> Then the distance traveled by the tire for the second is :

S = 7*42pi = 294pi inch per second.

Now we need to find the speed in terms of miles per hour.

==> We know that 1 hour = 60*60= 3600 seconds.

==> Also, we know that 1 mile = 5280 feet = 63,360 inch.

Now we will convert

294pi in/s = 294*3.14 * 3600/63360 = 52.48 mph

Then, the speed of the truck is 52.48 mph