Start by converting 300 feet to miles and 2.92 seconds to hours.
300 feet 1 mile
----------- x -------------- = ?
1 5280 feet
2.92 sec 1 hr
----------------- * ---------------- = ?
1 3600 sec
Reduce each of the above fractions. Then convert 300 feet
-------------- to the
2.92 sec
equivalent speed / rate in mph using the converstion factors you derived earlier.