1. 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 $\frac{9}{35}$

Find the possible values of $N$.
Show clear algebraic working.