The key to answering this question is considering that:
The distance is the product of speed and time.
In this case, we have:
distance = Speed x time or
d = s x t
And also
Speed = distance/time
s = d/t
We need to convert minutes into hours:
45 minutes * (1 h /60minutes) = 0.75 h
(We already have this value in the question)
Then, the distance is:
d = (3.4 miles/hour) * 0.75 h
d = 2.55 miles.
Therefore
If Kelly ran for 45 minutes, she completed a distance of 2.55 miles at this speed (3.4 miles/hour).