Yann will be x years old y years from now. How old was he z years ago?

Here we can change the variables to real numbers, just to make things easier for us. For the sake of calculation, let's say that x=10, y=6, and z=1.

So, y years from now, Yann will be x years old. If we plug in our randomly chosen numbers, we get that Yann will be 10 years old 6 years from now. This means that today, he would be x-y years old, or 10-6, so 4 years old today.

Next we ask how old he was z years ago, or in this case, 1 year ago. We know that if you're 4 years old now, you were 3 years old last year. In other words, you're 4-1 years old, or x-y-z years old.

Your answer is x-y-z.