$\begin{align}  & {{451}_{6}}-{{P}_{7}}={{305}_{6}} \\ & {{P}_{7}}={{451}_{6}}-{{305}_{6}} \\ & \text{       }{{451}_{6}} \\ & \text{    }\underline{\,-{{305}_{6}}}\text{   } \\ & \text{    }\underline{\text{    }{{142}_{6}}} \\ & \text{Convert 14}{{\text{2}}_{6}}\text{ to decimal system} \\ & {{142}_{6}}=1\times {{6}^{2}}+4\times {{6}^{1}}+2\times {{6}^{o}} \\ & \text{       }=36+24+2={{62}_{10}} \\ & \text{Convert }{{62}_{10}}\text{ to base 7 gives 11}{{\text{6}}_{7}} \\\end{align}$