Data Sufficiency

Could someone explain how you get from the first line to the second line or a good resource for understanding cross multiplication a little better? Thanks.

Hi dlencz:

To go from first line to the second, you have to multiply both the left and right hand side by the product of the denominators of the two sides. One can only do this when the product of the two is positive which means it is greater than 0. You can always do this without changing the sign, as long as the the factor you are multiplying both sides with is > 0. Luckily, in this case (b-k)(b+k)>0 is given.

Thing to watch
If the product was negative i.e. if (b-k)(b+k) < 0, then the inequality changes sign.