Oh Ivy -- I admire your DD! I hope the teacher takes it as politely as your daughter wrote her answer.

BTW, how is this solved using a model? Anything I come up with seems so messy and confusing!

Well, using the Singapore method, the bar model would look like a long narrow horizontal rectangle, which you divide into halves with a small vertical line in the middle. Then you label the complete long bar "6", indicating that it is 6 units long, and each of the small sub-bars "3", indicating that they are 3 units long.

My feeble attempt at a bar model:

.________6________

[________]________]

. . . .3 . . . . . 3