you typoed but apart from that have pretty much written the answer on the first line.

this is the equation of a line which you can plot. if you want to find two points to connect, try substituting values:

this gives you a point

this gives you a point

in the following, i used C as quantity of CDs and B as quantity of books.

plot 10C + 20B = 100 - Wolfram|Alpha

the wolfram plot goes into negative values. you probably should not do this.