The only way I can make sense of this is that the "al" are a mistype and you meant only:
A=1/2n
solve for n
let me know if i'm wrong!
but if I;m corret, you have to mupliply both sides by 2:
2A=\(\frac{2}{n}\)n
which ich: 2A=n
so the solution, after inverting the sides is:
n=2A.