(

) Take z = g = n. Then z y - r z y + r X = z (y - r y + r x) and this gives X = z x.

(

g) Take X = g x. Then g y - r(g y - g x)) = n (y - r (y - x)), so that n = g.

(

n) Take X = n x. Then

g y - r(g y - n x)) = n (y - r (y - x))