For (b^2-10b+21), start with listing the factors that multiply to give 21.
As 21 is positive and -10b is negative, you will find that the factors are in the following form:
where x and y are chosen from the pair of factors. If you multiply out the factors, you will get
Thus, you will choose from the list of factors a pair which will give
xy=21 and x+y=10.
Try the same for the denominator. It has more factors, so the list of factors will look like:
The method to follow is exactly the same as the numerator.
Tell us what you found.