The abscissa of h' is a,

CH'=|a-(- 1)|

Let the abscissa of point B be X, then the abscissa of point H is X,

Then CH=|x-(- 1)|

CH'=2*CH

|a+ 1|=2*|x+ 1|

If a>- 1, then X.

If a=- 1, then x =-1;

If a<- 1 and x>- 1, then x=-(a+3)/2.

Combining the above three, x=-(a+3)/2.