Answer:

Step-by-step explanation:
Move the b over to the other side to get the 2a alone.
2a=c-b
Then divide the entire equation by 2 to get a instead of 2a.


You are the correct one. If the person is not able to proof, and if you are rather sure about your answer, it is better to take your own answer instead :)
-9d = 63
<Multiply both sides by (-1)>
=> (-1) × (-9d) = (-1) × 63
=> 9d = -63
=> d = -7