Let p for volumn of pen, b for volumn of bundle of ink and pen, such that b= p+i, here i for volumn of ink.
The pen's price is 1, the bundle's price is 2. Suppose budget constrain is c, so we obtain c=p+2b, which induce c=3p+2b.
The x-axis indicate volume of pen, such that the slop of budget constrain line is -3/2.