Set each piece = to 56, assuming that the 2 variables for which you are not solving are each equal to 0.
7x -2(0)-14(0) = 56
7x=56 (divide each side by 7)
X = ?
(__, 0, 0)
7(0) - 2y - 14 (0) = 56
-2y = 56 (divide each side by -2)
Y= ?
(0, __ , 0)
7(0) - 2(0) - 14z = 56
-14z = 56 (divide each side by -14)
z = ?
(0, 0, __)
You would take the midpoint of one of the sides (3) and do the pythagorean theorem to find the height.
a^2+b^2=c^2
3^2+x^2=6^2
9+x^2=36
x^2=27
x=5.196 cm or 3sqrt(3)