1003124006

Find the value of a parameter $a\in \mathbb{R}$ so that the point $D=[-2;1;1]$ lies on the straight line $p$ defined by the parametric equations $$\begin{array}{rl}p:x& =1+m,\\ y& =-2+m,\\ z& =a+m;m\in \mathbb{R}.\end{array}$$