Answer:
y = 7
Step-by-step explanation:
Matthew should have subtracted 8 from both sides of the equation. Instead he has subtracted 8 from the left side and added 8 to the right side.
The correct way to solve this problem is:
y + 8 = 15
subtract 8 from both sides: y + 8 - 8 = 15 - 8
y = 7