Question: Did Leslier fly faster than 200 miles per hour?
$$Average=\frac{dis\tan ce}{time\ taken}$$
So, is average speed > 200 mph?
Statement 1: The distance from Hartfield to Weatherford is 470 miles.
Distance = 470 miles
Time taken = ??
Since the time taken is unknown, we cannot find the average speed. Hence, statement 1 is NOT SUFFICIENT.

b]Statement 2:[/b] The flight from Hartfield to Weatherford took Leslier exactly 2 hours.
Time taken 2 hours
Distance = ??
But we don't know the distance (average speed) covered by Leslier, so, statement 2 is NOT SUFFICIENT.

b]Combining both statements together;[/b]
Statement 1: distance = 470 miles
Statement 2: time taken = 2 hours
$$Average\ speed=\frac{dis\tan ce}{time\ taken}=\frac{470miles}{2\ hours}$$
$$Average\ speed=235\ miles\ per\ hour$$
Average speed > 200mph, so, Leslie flew faster than 200 miles per hours.
Therefore, both statements combined together are SUFFICIENT. The correct option is option C.