Marta walked 3.9 miles per hour for. 72. How far did she walk

Sadly, you neglected to include the unit of the 0.72.

Here are several possibilities:

0.72 seconds = = > 0.00078 mile

0.72 minutes = = > 0.0468 mile

0.72 hours = = > 2.808 miles

0.72 days = = > 67.392 miles

0.72 weeks = = > 471.74 miles

0.72 fortnights = = > 943.49 miles

0.72 years = = > 24,614.9 miles

0.72 millenia = = > 24,614,928 miles.