In the given molecular formula $$\mathrm{Be_n Al_2Si_6O_{18}}$$, according to charge balance in a molecule, we get

$$2n+2(+3)+6(+4)-18(2)=0$$

$$\Rightarrow n=3$$

So, the formula is $$\mathrm{Be_3 Al_2Si_6O_{18}}$$.