problem 2.9.4

Thank you for your work!

Just found your problem 2.9.4 is missing, I think the answer should be:

```
f(x, y) => 
f(x-1, x*y) => 
f(x-2, (x-1)*x*y) => 
f(x-3, (x-2)*(x-1)*x*y) => ... => 
f(1, 2*3*...*(x-2)*(x-1)*x*y) =>
f(0, 2*3*...*(x-2)*(x-1)*x*y) = (x!)*y

mathematically it's the factorial of x times y
```