Answer:
Height of the box = 11.5 in
Step-by-step explanation:
Let h be the height of the box.
Assuming the volume of the Box is  .
.
Given:
Length = Height - 4 = h - 4
Width = 3 in
We need to find the height of the box.
Solution:
We know that the volume of the box.

Substitute all given value in above formula.

Rewrite the equation as:



whole equation divided by 3.

Use quadratic formula with  
 

Put these a, b and c value in above equation.




For positive sign
 
  
h = 11.5 in
For negative sign

h = -7.5
We take positive value of h. 
Therefore, the height of the box h = 11.5 in