Question regarding squared norm?

Ugh this sounds confusing. Its been a while since I looked at stuff like this but I know that ||SX||^2 = ||SX|| ||SX|| so your first attempt is a bit off. Now SX is a matrix by a vector so you could try multiplying those out to get a column vector. Then try summing them or something and then square. Sorry I couldnt be clearer. Best of luck.