Your periodic interest rate is 0.07/12 = 0.005833333.
You save $100 at the beginning of each month.
When is the interest posted? Let's say at the end of the month, before you make your next savings deposit.
So at the beginning of month 1, you have $100.
At the end of month 1, you have $100 times 1.005833333, or $100.58.
At the beginning of month 2, you have $100.58 plus your new deposit, giving you $200.58.
At the end of month 2, you have $200.58 times 1.005833333, or $201.75.
Rinse and repeat until you get to the end of month 24.
Now, I don't know what assumptions are made about where the fractions of pennies go, or out to how many decimals the balance is kept. Answers may be off by a few cents if my assumptions aren't the same as yours. (I'm resolving the balance to the nearer cent when the interest is posted at the end of the month.)