The correct answer is 1/6.
In order to get the correct answer, you have to give them the same denominator. To do so, start by multiplying the first fraction by 3/3
1/2 * 3/3 = 3/6
Now multiply the second fraction by 2/2
-1/3 * 2/2 = -2/6
Now we can subtract.
3/6 - 2/6 = 1/6