PEMDAS
(parenthesis, exponents, multiplication, division, addition, and subtraction).
So, the first thing you'd do is evaluate the exponent, since there's no parenthesis. So, it would turn to
12-8-4 (2^3 = 2 * 2 * 2)
The next would be subtraction, since there's no multiplication, division, nor addition. Since there's only multiplication, it doesn't matter which one you do fist. So, it turns to
12-8-4
4-4
0
So, the answer would be 0.
P.S. Remember PEMDAS, it's awesome ;)