Compute the product using (a) the definition where Ax is the linear combination of the columns of A using the corresponding entries in x as weights, and (b) the row-vector rule for computing Ax. If a product is undefined, explain why. Set up the linear combination of the columns of A using the corresponding entries in x as weights. Select the correct choice below and, if necessary, fill in any answer boxes to complete your choice. (Type the factors and terms of your expression with the column matrices in the same order as they appear in the original matrix. In each term, type first the coefficient and then the column matrix.) The matrix-vector Ax is not defined because the number of columns in matrix A does not match the number of entries in the vector x. The matrix-vector Ax is not defined because the number of rows in matrix A does not match the number of entries in the vector x.