A=Pe^rt

$${\mathtt{A}} = {{\mathtt{Pe}}}^{{\mathtt{rt}}} = {\mathtt{A}} = {{\left(\underset{{\tiny{\text{Error: Unknown Identifier}}}}{{\mathtt{Pe}}}\right)}}^{\underset{{\tiny{\text{Error: Unknown Identifier}}}}{{\mathtt{rt}}}}$$

You cannot.

you can if you have values.  For example

A=1000*e^(0.05*4)

$${\mathtt{A}} = {\mathtt{1\,000}}{\mathtt{\,\times\,}}{{\mathtt{e}}}^{\left({\mathtt{0.05}}{\mathtt{\,\times\,}}{\mathtt{4}}\right)} = {\mathtt{A}} = {\mathtt{1\,221.402\: \!758\: \!160\: \!169\: \!9}}$$