How can I calculate and pay Zakat for past years in which I did not do so, given that my salary has changed over the years and there are amounts I spent whose exact measure I do not remember, knowing that the amount I currently have is 121,604 Dirhams?

For Zakat (definition) to be obligatory on money, two conditions must be met: reaching the nisab (definition) (the equivalent of 85 grams of gold or 595 grams of silver) and the passing of a full lunar year (hawl). Whoever possesses scattered amounts of money has two options: either assign a separate hawl for each amount and pay its Zakat when its hawl is due, or pay Zakat on all the money he possesses when the hawl of the first nisab he owned from it is due. The latter is easier and greater in reward. As for past years in which Zakat became obligatory, it is incumbent to pay it by estimating the wealth one possessed, excluding expenses.