-4x(2x-9)

-4x(2x-9)      distribute -4x over the terms in the parentheses

-8x² + 36x

$${\mathtt{\,-\,}}\left({\mathtt{4}}{\mathtt{\,\times\,}}\left({\mathtt{2}}{\mathtt{\,\times\,}}{\mathtt{x}}{\mathtt{\,-\,}}{\mathtt{9}}\right)\right) = {\mathtt{\,-\,}}\left({\mathtt{\,-\,}}{\mathtt{36}}{\mathtt{\,\small\textbf+\,}}{\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{2}}{\mathtt{\,\times\,}}\left(\underset{{\tiny{\text{Error: Unknown Identifier}}}}{{\mathtt{x}}}\right)\right)$$