For this case we have that by definition, the volume of a rectangular prism is given by:

Where:
It is the area of the base
h: It is the height
So:

The volume of the prism is:

Now, the volume of the cubes is:

Thus, we divide and find the number of cubes to fill the prism:

ANswer:
n = 2.5
Multiply the first bracket by -2
Multiply the second bracket by 2
x-3-12+4x= 4x-10
- negative number times + positive number= - negative number
- negative number times -negative number = + positive number
x+ 4x-3-12= 4x-10
5x-15= 4x-10
Move 4x to the other side
5x-4x-15= 4x-4x-10
x-15= -10
Move -15 to the other side
x-15+15= -10+15
x= 5
Answer : x= 5
You didn't write the (-2/7) as a power; you were supposed to do b to the power of -2 over 7, which is what the other person done in your other post regarding this