Question

Define balance. Explain the difference between formal balance and structural balance.

Answer 1

Balance is the composition of a piece; the negative and positive spaces that make the piece visually important. 
Formal balance is when everything is symmetrical. Think of a butterfly. When you split him in half, his left wing will math his right wing. 
Structural balance (I think your teacher means informal balance) is when the piece has elements or objects placed in a complex way, so opposite of formal balance. 

Answer 2

Answer:

Balance is the arrangement of elements within a piece so that no part overpowers another. Formal balance occurs when a piece has linear symmetry, in other words, if it was divided in half, one side would mirror the other. Structural balance includes symmetry, but also involves ensuring a piece, usually a building structure, is both stable and aesthetically pleasing to the eye.

Explanation: