Answer:
Remember that if u is a solution of the linear system Ax=b then satisfies that Au=b. Then, since [tex]x\in S[/tex], [tex]Ax=b[/tex], and since [tex]u\in S_0[/tex] then [tex]Au=0[/tex].
Now we need show that A(x+u)=b. Observe that [tex]A(x+u)=Ax+Au=b+0=b[/tex], this implies that [tex]x+u\in S[/tex]