Division of polynomials: answers not matching

[myself5]

On Elementary Algebra page 243 problem 72, it says to divide 9x^2 – 9x – 28 by 3x+4. The answer they gave was 3x-7 with no remainder. My answer was 3x+ 21 with a remainder of 84 - 112. This is what I did:

Code: Select all

        3x + 21
     -------------
3x+4) 9x^2 – 9x – 28
     -9x^2 – 12x
     -------------
           - 21x – 28
           - 63x + 84
           -------------
              84 – 112

This is at least the third time I have had this problem in the last page and a half. Can anyone see where I went wrong?

[stapel_eliz]

...divide 9x^2 – 9x – 28 by 3x+4.

I'm not sure what you're doing in your second multiplication and subtraction...?

You did the first multiplication and subtraction, and ended up with -21x - 28 as your new dividend. Why did you multiply the 3x by -21 instead of by -7...?

Eliz.

[myself5]

You did the first multiplication and subtraction, and ended up with -21x - 28 as your new dividend. Why did you multiply the 3x by -21 instead of by -7...?

Yeah, it appears I was using the wrong number. I think I have it figured out now. Thanks so much for your help!