Can you please explain me Question 3 I found some answers fo

Can you please explain me Question 3? I found some answers for that through google but I don\'t understand them. Can you make it clear for me?

Solution

if there are two different solutions then there are infinitely many different solutions. let xand y are two different solutions i.e. Ax = Ay = b then A(x-y) = 0 so A(t (x-y)) =0 for any real t and hence Au = b where u = x+ t(x-y)