To prove that the answer is 2, you must also prove that it's impossible to write 48÷2(9+3) as:
48 / 2 * (9+3) = 48 / 2 * 12
From the link:
http://www.mathsisfun.com/operation-order-bodmas.html
Otherwise just go left to right. Example:
Right:
30 ÷ 5 × 3 = 6 × 3 = 18
Wrong
30 ÷ 5 × 3 = 30 ÷ 15 = 2

