Multiply the rate of change of water lever with respect to time times the total amount of time to figure out the total change in water level:
![(-0.21\cdot\frac{\operatorname{cm}}{day})\cdot(32.1\text{ days)=-6.741 cm}]()
Round the answer up to 2 significant figures:
![-6.741\operatorname{cm}\approx-6.7\operatorname{cm}]()
Therefore, the total change in the pool's water level was -6.7 cm.