Alice plans to buy a house. She has $2,000 in her savings account.
She plans to contribute $150 to the account each month.
We know that with each month, n, she deposits $150 and so, each month, her savings account balance increases by $150.
This simply means that after n months, her savings account balance would have increased by:
150 * n = 150n
Her initial balance was $2000, therefore, her balance after n months will be the sum of 150n and the amount that was initially there.
That is:
f(n) = 2000 + 150n
This is the function that describes the relationship between the number of months and her balance.