One cold night the temperature was 0 degrees at 8 p. m. the temperature dropped at a constant rate from 8 p. m. to midnight. at midnight the temperature was −16.8 degrees.

what was the temperature at 9 p. m.?

Let y be the temperature
Let m be the rate of temperature drop in degrees per hour
Let x be the number of hours elapsed since 8PM
The initial temperature (at x = 0) is 0
y = mx
Four hours from 8 PM, the temperature was -16.8 degrees
-16.8 = m(4)
m = -4.2 degrees per hour
At 9PM, one hour will have elapsed.
y = -4.2(1)
y = -4.2 degrees