Question 1005646
The diagonal distance from the lower front left corner to the upper back right corner is what you want...
d^2 = a^2 + b^2 + c^2
d^2 = 16^2 + 12^2 + 8^2
d^2 = 256 + 144 + 64
d^2 = 464
d = 21.54 cm