determining plane slopes

[gib65]

How can I determine the slope of a plane? Then, how can I set the slope of another plane to match the first one?

[fuccaro]

Time for some math!
The following program will prompt the user for 3 points and it will draw the intersection line between the plane passing through those points and a horizontal plane. Just draw a perpendicular line from any point contained in the plane to the line returned by the program.
I wrote the program without error trap so please enter 3 different, non collinear points.

;| Intersection between a user defined plane and a horisontal one
   the 20th of sept 2007    mfuccaro@hotmail.com
|;
(defun c:intpl( / q1 q2 q3 l z0 a b c d) ;INTersection of PLanes
 (defun d3(l)
  (- (+ (* (nth 0 l) (nth 4 l) (nth 8 l))
    (* (nth 2 l) (nth 3 l) (nth 7 l))
    (* (nth 1 l) (nth 5 l) (nth 6 l)))
     (+ (* (nth 2 l) (nth 4 l) (nth 6 l))
    (* (nth 0 l) (nth 5 l) (nth 7 l))
    (* (nth 1 l) (nth 3 l) (nth 8 l))))
  )
  (setq l (list (setq q1 (getpoint "first point"))
        (setq q2 (getpoint "2nd"))
        (setq q3 (getpoint "3rd")))
    z0 (apply 'min (mapcar 'caddr l))
    a (d3 (append '(1) (cdr q1) '(1) (cdr q2) '(1) (cdr q3)))
    b (d3 (list (car q1) '1 (last q1) (car q2) '1 (last q2) (car q3) '1 (last q3)))
    c (d3 (list (car q1) (cadr q1) '1 (car q2) (cadr q2) '1 (car q3) (cadr q3) '1))
    d (- (d3 (append q1 q2 q3))))
  (if (zerop b)
    (entmake (list '(0 . "LINE") (list 10 (car q1) (cadr q1) z0) (list 11 (car q2) (cadr q2) z0)))
    (entmake (list '(0 . "LINE")
           (list 10 (apply 'min (mapcar 'car l))
             (- (/ (+ (* a (apply 'min (mapcar 'car l))) (* c z0) d) b)) z0)
           (list 11 (apply 'max (mapcar 'car l))
             (- (/ (+ (* a (apply 'max (mapcar 'car l))) (* c z0) d) b))
             z0)))
    )
  )

As about the second question: try the ALIGN command

[Alan Cullen]

It would also help if you specified your autocad version.