"Hint: Show that if X1 and X2 are different solutions to AX = B, then X1 + c(X2 - X1) is also a solution, for every real number c. Also, show that these solutions are different."
I think I know how to show they are different. If X2 is different from X1 then X2 - X1 =/= 0. Therefore X1 + c(X2 -X1) =/= X1. (except for the one case when c = 0)
However I don't know how to prove that X1 + c(X2 - X1) is also a solution.