Rotate parent object so that a child faces mouse


Hi,
How can i rotate the parent (root) object so that a child faces a given point?
In the picture above, i want to rotate the “Root” object so that “Child2” faces the “X”.
I understand there can be cases where it is not possible. If “Child1” is to long, for example.
From the direction vectors (in green in the picture) I cannot figure out a simple solution. Thanks.