The number of hours since the bakery opened, plus the number of remaining boxes is equal to 42.
x + y = 42
So the number of boxes remaining is equal to y, and
y = 42 - x, the number of hours the bakery was open.
So at the start of the day, the bakery was open for 0 hours, and y, the number of boxes "remaining" at the start of the day, will be equal to 42 - 0 = 42.