In the absence of upstream flood injection, the reservoir water level will decrease by a meter/hour at a uniform speed by opening the floodgates, and increase by b meters/hour at a uniform speed by setting that the upstream flood will not open the floodgates in flood season.

So from the meaning of the question:

3a(b-a)+h=0

10(b-2a)+h=0

Solution:

a=2b，h=30b

Assuming that all six gates are open, the water level relative to the warning line after 3 hours is

(b-na)x+h =(b- 12b)×3+30b =-3b。

Because b > 0 and -3b < 0, it means that the water level is lower than the warning line. Therefore, the reservoir can reduce the water level to the warning line within 3 hours.