Answer:
14 in and 16 in
Explanation:
Given data
We are told that the aquarium has the shape of a cube
the first specification is 
2744 cubic inches
volume= 2744 cubic inches
length = ∛2744
length= 14 in
the second specification is 
4096 cubic inches
volume= 4096 cubic inches
length = ∛4096
length= 16 in
Hence the possible lengths are 
14 in and 16 in