It depends entirely on where you put the parentheses (). If they are not shown, then you follow the order that the digits are in, therefore the answer is 5:

5 + 2 = 7 x 2 = 14 - 9 = 5

With parenthses around the (2 x 2) part, the answer becomes zero:

5 + 4 - 9 = 0

.

"I told the doctor I broke my leg in two places. He told me to quit going to those places". - Henny Youngman