Dividing and signs, step by step

−(x − 7) becomes −x + 7, and the second sign is the one that gets left behind.

The question

Noor wrote 3(x + 7) − (x − 9) = 2x + 30. Which is correct?

3(x + 7) − (x − 9)

3(x + 7) − (x − 9)=3x + 21 − x + 9

3(x + 7) − (x − 9)=2x + 30

they wrote:2x + 30

Do not check by reading their line. Somebody else's working is the most persuasive thing on the page, and rereading it is how you end up agreeing with it — do the step yourself first, then compare.
On your own, and writing the step nobody writes down: 3(x + 7) − (x − 9) becomes 3x + 21 − x + 9.
Tidy that up and it is 2x + 30.
Now compare — and this time the two agree. The line on the page was already right, which is a finding, not a missing mistake. Some of them are.

How it works.

01
Do not check by reading their line. Somebody else's working is the most persuasive thing on the page, and rereading it is how you end up agreeing with it — do the step yourself first, then compare.
02
On your own, and writing the step nobody writes down: 3(x + 7) − (x − 9) becomes 3x + 21 − x + 9.
03
Tidy that up and it is 2x + 30.
04
Now compare — and this time the two agree. The line on the page was already right, which is a finding, not a missing mistake. Some of them are.