Y is the total number of balls depending on the number of Saturdays
x is the number of Saturdays and is multiplied by 5 since for each Saturday it acquires 5 balls
1200 is the current collection that he has since the balls he acquires will be added to this value
we replace x by the number of Saturdays in a year, that is, 52
in a year will have 1460 balls
the domain represents the values that x can take
x can take values from 0 to infinity, but only integer numbers ,It cannot take negative values because it is impossible to have for example -2 Saturdays
so the domain is
[0,inf]
the set notation is