Courtesy of Heureka :)
(t−1)3+6(t−1)2+12(t−1)+8=(t−1)3+3(t−1)2+3(t−1)+1+3(t−1)2+9(t−1)+7=t3+3(t−1)2+9(t−1)+7=t3+3(t2−2t+1)+9t−9+7=t3+3t2−6t+3+9t−9+7=t3+3t2+3t+1|(1+t)3=t3+3t2+3t+1=(1+t)3
\begin{array}{|rcll|} \hline && \mathbf{(t-1)^3 + 6(t-1)^2 + 12(t-1) + 8 } \\
&=& (t-1)^3 + 3(t-1)^2+3(t-1) +1\\
&& \quad + 3(t-1)^2+9(t-1)+7 \\
&=& t^3 + 3(t-1)^2+9(t-1)+7 \\
&=& t^3 + 3(t^2-2t+1)+9t-9+7 \\
&=& t^3 + 3t^2-6t+3+9t-9+7 \\
&=& t^3 + 3t^2+3t+1 \quad & | \quad (1+t)^3 = t^3+3t^2+3t+1\\
&\mathbf{=}& \mathbf{(1+t)^3} \\
\hline \end{array}