she starts with 2000 pounds.
it doesn't matter what the value of n is, she will have at least 2000 pounds in the bank.
that becomes your fixed component of the equation.
when n = 0, what is in the account is 2000 pounds.
for each week after that, she deposits 120 pounds.
that becomes your incremental component of the equation.
that depends on the value of n.
when n = 0 there is no additional amount added to the account.
for each n thereafter an additional 120 pounds is added to the acocunt.
the total added is equal to 120 * n
after n weeks, 120 * n has been added into the account.
for example:
when n = 3, the account balance is as follows:
n balance
0 2000
1 2000 + 120 = 2120
2 2120 + 120 = 2240
3 2240 + 120 = 2360
the equation is 2000 + 120 * n
that becomes 2000 + 120 * 3
that becomes 2000 + 360
that becomes 2360
the equation correctly determine what the balance in the account will be after n weeks.