Pippa has a box containing N pens. There are only black pens and red pens in the box.
The number of black pens in the box is 3 more than the number of red pens.
Pippa is going to take at random 2 pens from the box.
The probability that she will take a black pen followed by a red pen is
9
35
Find the possible values of N.
Show clear algebraic working.