Right, you have this linear map, denoted L, and you have said what it does. You now wish to find its kernel. That is, which matrices M are such that L(M)=0? That is, for which values of are such that ?
(I now have to go, sorry!)
With all due respect I think the OP is way more lost in this that what we could have guessed at the beginning, and I think he doesn't need help in this question but rather a thorough check over the whole material of this course, perhaps from the start...and the sooner the better, as things are going to get tougher pretty quick and without the elementary foundations is almost impossible to succeed.
I don't believe that a "thorough review" of all this stuff can be given within the frame of MHF but rather, imo, with the help of some private tutoring.
To prove two vector spaces and are isomorphic you must
(1) Find a linear map between them, ,
(2) Prove that is an injection,
(3) Prove that is a surjection.
In some ways, step (1) is the hardest - it requires some imagination. For the question you are looking at, you have done this;
So this is your linear map. You must, however, prove that it is linear. That is, you have to prove that
For (2), is an injection if and only if , by definition. Now, as your map is a linear map, . Therefore, is an injection if and only if the set consists only of the zero of the linear space (as then ). This set is called the kernel of your linear map, .
Thus, what you need to prove is that if then .
Does that make sense?
Now, for (3) it is sufficient to prove that each of the generators of the space is mapped onto by some element of (why?). So, what are the generators of in this case, and which elements of are mapped to them?
I would agree with Tonio, you do need proper help. We can, and are willing, to help you here. However, you need to give more constructive replies. `I'm confused' doesn't really count as constructive. You have to say why you are confused.