Answer:
Line E
Explanation:
Given
The instructions in the question
Required
The first line of mistake
The question has missing details, as the original expression is not given;
Reading through the question, one can observe that, Mary made a mistake at Line E.
The expression on D is:

The expression on E is:

The actual result of D is:

not

Because:


With the available information, we can conclude that Mary's first mistake is at line E