the process is called completing the square ...
$\displaystyle -3(x^2 + 4x) - 1$
$\displaystyle \color{red}{-3}\color{black}{(x^2 + 4x} \color{red}{+4}\color{black}{) - 1} \color{blue}{+ 12}$
adding -3(4) is equivalent to adding -12 to the expression ... adding +12 balances the value of the expression.