Keeping lines straight can be tricky

hello,

i had same problem freehand after macromedia took them over(not before). have straight line vertical. top anchor , bottom anchor, b. add third anchor onto line, c (in-between & b). create unspecified angle a, drag c right of line ab, down same x axis b, creating straight line between b & c. without guide, when let go of c, line between b , c kinks.  far away, looks straight prints jagged. can't seem keep anchor c moving off x axis of b. if sounds understandable, know how keep anchor moving when let go of mouse? or key stroke keep line straight?

 

below exaggerated illustration of idea.

 

be don't have align pixel grid selected on transform panel.

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