I want to try to find the largest prime twin under a given number using TI Basic on a Nspire.

I.e. if the number is 8 the program should return:

5, 7

I thought it should be something like this:

```
Define grpr2(x)=
Prgm
If isPrime(x-1) -> a
Then isPrime(x-3) -> b
Else (x-1) -> x
Endif
Disp a, b
EndPrgm
```

Obviously I'm doing something wrong. I think it has to do with defining a and b, but I have no idea what. Can someone help me?