Answer:
I believe the answer to this question is: 6 7/12.
15- 10 divided by 21- -9 should give you your slope
X = 2V6 + 4 or x =-2V6 + 4
There were 20 erasers added to each box, because 15 plus 20 is 35. 85 plus 20 is 105, and 35 times 3 is 105. This means that there are 35 erasers in box B after adding 20, and 105 erasers in box A after adding 20. I hope this helps you!
The mystery number will be 19.4596
