Assume x cm taken off each side.
Reduced volume = (12 - x)(10 - x)(6 - x) cm3
= (120 - 22x + x2)(6 - x)
= 720 - 252x + 28x2 - x3
This must equal 12*10*6 - 400 = 720 - 400cm3
720 - 400 = 720 - 252x + 28x2 - x3
x3 - 28x2 + 252x - 400 = 0
Real number solution: x = 2 cm
i.e. 2 cm must be removed from each side.