What is the longest flagpole (in whole feet) that could be shipped in a box that measures 1ft by 2 ft by 12 ft?

Consider a pole running from one corner to the farthest corner of a box. 
In 3D space the start of the pole would be at (0,0,0).
The endpoint would be at (12,2,1).
Use the distance formula:
D^2 = (x1-x2)^2 + (y1-y2)^2 + (z1-z2)^2
D^2 = (12-0)^2 + (2-0)^2 + (1-0)^2
D^2 = 144 + 4 + 1
D^2= 149
D = sqrt (149) 
D = (approx.) 12.21 ft
D = 12 ft

Have a nice day :D