To determine what length board Mia should use for the length of the shelter, we first need to convert the length from feet to inches.
Mia wants the length of the shelter to be 3.5 feet. Since there are 12 inches in a foot, we calculate:
3.5 feet × 12 inches/foot = 42 inches.
Therefore, Mia should use a 42 inch board for the length of the shelter.
The correct response is: 42 inch board.