If you draw a graph of the temperature, you know two points
on the graph, (so naturally your graph is a straight line).
The two points are:
x = 10 hrs after midnight, y = 71°F
and
x = 15 hrs after midnight, y = 86°F
Whenever you have two points on a graph, the slope
of the line between them is
(y₂ - y₁) / (x₂ - x₁) .
Slope = (86° - 71°) / (15 hrs - 10 hrs) =
(15°) / (5 hours) = 3 degrees per hour .
What it means:
If the temperature was 71° at 10 AM and later it was 86° at 3 PM,
then from 10 AM until 3 PM the temperature increased at the
average rate of 3° per hour.