She uses 11/36 yards of string for each.
In this problem, you're dividing the amount of string by the number of banners, so 11/12 ÷ 3. To do this though, it's easiest to convert the 3 into a fraction. After you do this, you're left with:
11/12 ÷ 3/1
When dividing fractions, you flip the second fraction upside down and change the operation to its opposite, which gives you:
11/12 × 1/3
Then you just multiply across. 11×1=11 and 12×3=36, so you're left with 11/36 yards of string.
