You want to slove a matrix? Sorry. Valentines day was 3 days ago. Too late for s"love".
A is a column vector, composed of the elements [2 0 0 0 1].
B is a vector, used to index into the matrix (or vector) A.
What is the 5th element of A? Hint: Matlab uses a 1 based index system. So the 5th element of A is 1. Likewise, the 1st element of A is 1, etc.