G6.2 If $p-q=2 ; p-r=1$ and $b=(r-q)\left[(p-q)^{2}+(p-q)(p-r)+(p-r)^{2}\right]$. Find the value of $b$.
$\begin{aligned} b & =[p-q-(p-r)]\left[(p-q)^{2}+(p-q)(p-r)+(p-r)^{2}\right] \\ & =(2-1)\left[2^{2}+2 \cdot 1+1^{2}\right] \\ & =2^{3}-1^{3}=7\end{aligned}$