Well, the formula for simple interest is
"I" is interest, "p" is the principal, "r" is the rate, and "t" is the time.
If you substitute the numbers, you get
When you multiply them together, you get
So, the simple interest for 3.5 years is $210.
But, in order to get the total balance, you have to add the interest and the principal together.
So, the balance after 3.5 years is $1,210.