Emilio owns a bakery. the number of boxes of cookies he has left on a monday is represented by the function x + y = 42, where y is the number of boxes of cookies remaining in the bakery and x is the number of hours since he opened the bakery. how many boxes did he have at the start of business on monday? (note: x ≥ 0, y ≥ 0, and y takes only integer values.)

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.