Hey there! Hello!
For this problem, I'm going to assume that the 9% interest is a yearly rate. Please let me know if this assumption isn't correct, and we'll take it from there.
To calculate the full payment including the interest, all you need to do is multiply the $900 by 9%, or 0.09, to get $81. Since this is a yearly thing, we'll multiply 81 by 3 to represent 3 years, and we'll get $243. The total amount a customer will have to pay for the computer is $1,143, which is the interest plus the original price.
Now, for the monthly rate, you would need to divide that amount by 36, which is how many months are in three years. In order to pay the amount for the computer off in three years, a customer would have to pay $31.75 per month.
I hope this helped you out! Feel free to ask any additional questions if you have any. :-)