She made the mistake of grouping unlike terms and factorizing.
Given that
Helene is finding the sum (9 + 10i) + (–8 + 11i).
She rewrites the sum as (–8 + 11)i + (9 + 10)i.
We have to determine
Which statement explains the property of addition that she made an error in using?
According to the question
The mistake she did is in the second term distributing.
(9+10i) is not equal to (9+10)i
Similarly (-8+11i) is not equal to (-8+11)i.
The correct method she should have done is given below;
Grouping real terms together and imaginary terms together and finding the sum is,

Hence, she made the mistake of grouping unlike terms and factorizing.
To know more about Complex Number click the link given below.
brainly.com/question/10078818
Answer:
I KNOW AND I AM PUMPED
Step-by-step explanation:
The answer is 882 because if you take answer choices A, B, and C, and divide them by 6, none of them come out to a whole number. However, D, 882, comes out to 882/6= 147. Since it is a whole number, 882 is a multiple of 6.