I have the following :

1. $\displaystyle a_{1} | a$
2. $\displaystyle b_{1} | b$
3. $\displaystyle a_{1}b_{1} | ab$
4. $\displaystyle a_{1}b_{1} | k$
5. $\displaystyle a | k$
6. $\displaystyle k | ab$
7. $\displaystyle k | aa_{1}b_{1}$

I need to show that $\displaystyle k=ab_{1}$.
I try to show that $\displaystyle k | ab_{1}$ and $\displaystyle ab_{1} | k$, but I dont know how to show this.