Elena goes for a long walk. This graph shows her time and distance traveled throughout the walk.

Coordinate plane, x, time in hours, 0 to 5 by ones, y, distance in miles, 0 to 10 by ones. Line segments connect the origin to 1 comma 3, to 1 point 5 comma 3, to 2 comma 5, to 4 comma 9.

What was her fastest speed, in miles per hour? Type your answer in the box below.