The sum of all values of $a$, for which the points whose position vectors $\hat{i}-2 \hat{j}+3 \hat{k}, 2 \hat{i}-3 \hat{j}+4 \hat{k},(\alpha+1) \hat{i}+2 \hat{k}$ and $9 \hat{\mathrm{i}}+(\alpha-8) \hat{\mathrm{j}}+6 \hat{\mathrm{k}}$ are coplanar, is equal to
