Hello!
First you have to find the volume of the triangular prism
To find this you use the equation

h is height
a, b and c are the side lengths of the triangle
To find the side lengths you use the Pythagorean Theorem
The side lengths are about 7.2
The you put this into the equation to find the volume of a triangular prism
When you do this you get around 95.52
Subtract this from the total volume
3840 - 95.52 = 3744.48
Put this into the equation to find the volume of a rectangular prism
12 * 30 * x = 3744.48
Divide both sides by 12
30 * x = 312.04
Divide both sides by 30
x = 10.401
The answer is 10.401 meters
Hope this helps!