\((x-12)-(3x+21)=-1\)
\(\Rightarrow x-12-3x-21=-1\)
\(\Rightarrow x-3x-12-21=-1\)
\(\Rightarrow-2x-12-21=-1\)
\(\Rightarrow-2x-33=-1\)
\(\Rightarrow-2x=\left(-1\right)+33\)
\(\Rightarrow-2x=32\)
\(\Rightarrow x=32\div\left(-2\right)\)
\(\Rightarrow x=-16\)