Why can't the calculator calculate 0?

$${\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{x}} = {\mathtt{5}}{\mathtt{\,\times\,}}{\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{6}} \Rightarrow {\mathtt{x}} = {\mathtt{2}}$$

$${\mathtt{2}}{v} = {\mathtt{5}}{v}{\mathtt{\,-\,}}{\mathtt{6}} \Rightarrow {\mathtt{v}} = {\mathtt{2}}$$

$${\mathtt{v}} = {\mathtt{6}}{v}$$

$${\mathtt{v}} = {\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{v}}{\mathtt{\,\times\,}}{\mathtt{3}} \Rightarrow {\mathtt{v}} = {\mathtt{2}}{\mathtt{\,\times\,}}\left(\underset{{\tiny{\text{Error: Unknown Identifier}}}}{{\mathtt{v}}}\right){\mathtt{\,\times\,}}{\mathtt{3}}$$

You are right, the calculator won't do that.