if height is equal to 2.20m at exactly 9am, and then the height is equal to 1.7m at exactly 10am. At what approximate time will the height be equal to 1.71m
The height changed by -0.5m in one hour. That means that it was 1.71m slightly before it fell to 1.7m.
Its rate of change between 9 and 10 am was -0.5 m/hr.
So, it was .01 hours before 10 am when the height was 1.71m. .01 hrs = 0.6 minutes, or 36 seconds.
So, the height was 1.71m at 9:59:24
I suspect a typo, since you seemed to think that the desired time was in the future. Fix it and repeat the solution.