1) you have a 3x1 vector x of three i.i.d. Gaussian random variables.
you have a matrix A which is k x 3, where k=2, 3, 4, and in all cases the
coefficients of A are such that its rank is the minimum between k and 3.
Write the joint p.d.f. of y=Ax for al the values of k.
(Note: k=2,3 are easy but k=4 requires some thought)
2) Let x be a vector of variables, A a matrix and y a given vector. What is the solution of min_x ||Ax-y||^2
Can you put some indication of your own thought process? It seems like you just post your homework problems and ask for help without attempting. I am not saying you are but it sure seems like it. That is probably the reason why no one has reply yet.