Let x = number of tapes and y = number of CD's.
Your equation would look like this:
x + y = 13
8x + 9y = 110
The first thing you would do is change x + y = 13 to be x = 13 - y. So now you have:
x = 13 - y
8x + 9y =110
Now you insert the value of x (the first equation) into all the places where x is written (one place in the second equation). Basically, 8x would become 8(13 - y) or 104 - 8y. That would mean that now you have an equation that looks like this:
104 - 8y + 9y = 110
The reason why there isn't a second line anymore is because you already inserted x into this equation, meaning it's still there, just not in the form of two lines.
Now we add the y values:
104 + y = 110
And we subtract 104 from 110 so that:
y = 6
And that is your y. Now all we have to do to get x is to plug in y from our original equation, x + y = 13. So that would look like this:
x + 6 = 13
Now subtract 6 from 13 so that...
x = 7
And that's it! x = 7 and y = 6. If you remember from earlier, we let x be the number of tapes and y be the number of CD's, so in total Rene bought 7 tapes and 6 CD's.
You're welcome!
