The question is not hard at all: it's funny how some people don't get the similarity between this and the Pythagorean theorem!
The formula for right triangle: c^2 = a^2 + b^2
Formula for distance between opposite points of rectangular prism: d^2 = x^2 + y^2 + z^2
See how simple it is? It probably faster than the time it takes for me to open my front door! (Really, that creature is super unruly, maybe oil will help)
Do you really want an answer? An answer you want, an answer you get.
Using the formula, we see that the diagonal squared is:
d^2 = 3^2 + 4^2 + 12^2
3^2 + 4^2 + 12^2 = 9 + 14 + 144 = 167
Now root that:
And....your answer is sqrt(167)
maybe if you paid attention in class (I am assuming they gave a lecture) you would have known this simple formula. you could even do a quick google search!
maybe I finally entered my house after hours of trying to swing the door open...