Here you go, the area of any regular hexagon is A = 2.598(a x a) with "a" being the length of a side and A the area.
A=2.598(a x a)
A=2.598 (14 x 14)
A=509.2 square inches
Volume = Area x height V=A x h
V= 509.2 x 24
V = 12220.99 cubic inches
There are 231 cubic inches in a US gallon so…
V = 12220.99 / 231 = 52.9 Gallons.