I have a math problem, help!

The water raise to 9 cm told you the height of the water level after inserting the metal, therefore calculating the Volume of the water with metal will gives you further solution for the original Height of the water.
Let the height of the original water be H
Transform the length and width from meter to centimeter
0.4M = 40cm;
0.8M=80cm
The Volume of water with Metal = 40x80x9 = 28800 cm3
The Volume of water without Metal = 28800 - 6400 = 22400 cm3
The Volume of water without Metal = 40 x 80 x H = 3200H cm3
The Height of the water:
3200H = 22400
H = 7 cm
The original level of water (without metal) is 7cm
The water level raise after inserting the metal is 9cm - 7cm= 2cm

Qa)
Let H cm be the change in water level
6400 = (80)(40)H
H = 6400 / 3200
H = 2 cm
Therefore, there is a 2 cm change in water level.


Qb)
The original water level is:
= ( 9 - 2 ) cm
= 7 cm