$${{\mathtt{e}}}^{{\mathtt{\,-\,}}\left({\mathtt{0.5}}{\mathtt{\,\times\,}}{{\mathtt{x}}}^{{\mathtt{2}}}\right)}$$ => $${\mathtt{\,-\,}}\left({\mathtt{0.5}}{\mathtt{\,\times\,}}{{\mathtt{x}}}^{{\mathtt{2}}}{\mathtt{\,\times\,}}{log}_{10}\left({\mathtt{e}}\right)\right)$$
$${{\mathtt{3}}}^{\left({{\mathtt{x}}}^{{\mathtt{2}}}\right)} = {\mathtt{81}}$$ => $${{\mathtt{x}}}^{{\mathtt{2}}}{\mathtt{\,\times\,}}{log}_{10}\left({\mathtt{3}}\right) = {log}_{10}\left({\mathtt{81}}\right)$$ => $${{\mathtt{x}}}^{{\mathtt{2}}} = {\frac{{log}_{10}\left({\mathtt{81}}\right)}{{log}_{10}\left({\mathtt{3}}\right)}}$$
$${\mathtt{x}} = {\sqrt{{\frac{{log}_{10}\left({\mathtt{81}}\right)}{{log}_{10}\left({\mathtt{3}}\right)}}}} \Rightarrow {\mathtt{x}} = {\mathtt{2}}$$