Explanation:
Ignore the denominators for now.
The numerators -11 and 9 add up to -11+9 = -2
Then we place that result over the common denominator 21 to get the final answer -2/21
This trick only works if the denominators are all the same. If not, then you'll need to find the LCD first before you can add the fractions. Luckily, all of the denominators were the same at 21.
