Sum of the first two is:
-3x+5+7-4x = -7x+12
That subtracted from 5x+17:
5x+17-(-7x+12) = 12x+5
No error - A and B are completely right, and using the subtraction law D is right
The correct answer is letter C.
The reason is because there are 12 cake donuts out of the 36 total. This is a simple ratio of 1 to 3. This means one out of every 3 people would get a cake donut. If there are 12 people, this is 4 groups of 3, so one person from each group of 3 is 4 people.
